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quantum chromodynamics ( qcd ) is the theory of the hadronic interactions . it is an elegant theory whose full non perturbative solution has escaped our knowledge since its formulation more than 30 years ago.@xcite the theory is asymptotically free@xcite and confining.@xcite a particularly good test of our understanding of the nonperturbative aspects of qcd is to study particles where the gauge field plays a more important dynamical role than in the standard hadrons . in particular glueballs , bound states of gluons , represent such a scenario . the glueball spectrum has attracted much attention since the early days of qcd.@xcite the interest in this subject is related to the significant progress in the understanding of the properties of such states within qcd , as well as , in the new possibilities for their identification in modern experiments . historically the investigation of the glueball properties started in the bag model by jaffe and johnson.@xcite they found many glueball states with different quantum numbers lying in the mass interval 1000 - 2000 mev . they emphasized that one should expect rather small widths for such states because their decays in conventional hadrons violate the okubo - zweig - iizuka ( ozi ) rule.@xcite    after this pioneering work the study of glueballs was carried out by using various versions of constituent models , by exploiting the qcd sum rule approach and by performing lattice qcd calculations . glueballs have not been an easy subject to study due to the lack of phenomenological support and therefore much debate has been associated with their properties . the main achievement of these approaches is the understanding of the deep relation between the properties of the glueball states and the structure of the qcd vacuum . besides , they provide a determination of the spectrum both in gluodynamics , the theory with just gluons and no quarks , and in qcd . however , in ( unquenched ) qcd , the results of several calculations for the spectrum are still not universally accepted , in particular , for the lowest lying glueballs.@xcite    from the phenomenological point of view it has become clear by now that it is difficult to single out which states of the hadronic spectrum are glueballs because we lack the necessary knowledge to determine their decay properties . moreover the strong expected mixing between glueballs and quark states leads to a broadening of the possible glueball states which does not simplify their isolation . the wishful sharp resonances which would confer the glueball spectra the beauty and richness of the baryonic and mesonic spectra are lacking . this confusing picture has led to a loss of theoretical and experimental interest in these hadronic states . however , it is important to stress , that if they were to exist they would be a beautiful and unique consequence of qcd . at the present , several candidates for the low mass glueballs with quantum numbers @xmath0 , @xmath1 , @xmath2 and @xmath3 are under discussion . @xcite    in this review we will discuss the modern development in glueball spectroscopy from various perspectives . in section  [ sec : lattice ] we will summarize the results that lattice techniques have obtained for the spectrum , both in the pure gauge theory and in the unquenched calculations . in section  [ sec : models ] we present a review on constituent models . section  [ sec : sumrules ] is dedicated to discuss qcd sum rules . in section  [ sec : prodecay ] the production and decay mechanisms of glueballs in hadronic reactions are discussed . in section  [ sec : qgp ] the peculiarities of glueball production and behavior in quark - gluon plasma ( qgp ) is considered . section  [ sec : other ] is dedicated to present two open topics the relation between the pomeron and glueballs and glueball - quarkonium mixing . finally in section [ sec : conclusions ] we extract the main conclusions of our analysis and try to foresee future developments . gluon self - couplings in qcd suggest the existence of glueballs , bound states of mainly gluons . investigating glueball physics requires an intimate knowledge of the confining qcd vacuum and it is well known that such properties can not be obtained using standard perturbative techniques . to handle the nonperturbative regime of qcd , one can resort to numerical methods , known as lattice qcd . lattice qcd needs as input the quark masses and an overall scale , conventionally given by @xmath4 . then any green function can be evaluated by taking average of suitable combinations of lattice fields in vacuum samples . this allows masses and matrix elements , particularly those of weak or electromagnetic currents , to be studied . one limitation of the lattice approach is in exploring hadronic decays because the lattice , using euclidean time , has no concept of asymptotic states . lattice qcd was originally formulated by wilson  @xcite and is a clever implementation of the qcd dynamics using a finite difference formalism . the starting point is the correlation function in a discrete euclidean space @xmath5 where @xmath6 is the fermion action and @xmath7 the pure gauge action . the continuum limit is controlled by the input parameter @xmath8 ( @xmath9 is the number of color ) . by varying the inverse lattice coupling @xmath10 we vary the lattice spacing @xmath11 . the fermion action can be integrated out exactly in eq . to produce the fermion determinant . the determinant describes the dynamics of the sea quarks . in quenched qcd calculations , the determinant is set to a constant.@xcite    the physics is extracted from the fit @xmath12 @xmath13 are the energy eigenstates and @xmath14 the corresponding masses . in practice , one has to choose the operator @xmath15 which best overlaps with the lowest - lying glueball in the channel of interest . @xmath15 is thus expanded in a basis with well - defined symmetry properties under the octahedral group and the variational coefficients are determined by monte - carlo simulations.@xcite    the spectrum in a box with periodic boundary conditions includes not only single glueball states , but also states consisting of several glueballs and torelons ( gluon excitations which wrap around the toroidal lattice ) . fortunately , torelons are found to overlap only weakly with single glueball states.@xcite    since ( classical ) gluodynamics is dimensionless , its observables will be also dimensionless . masses are usually expressed in terms of the string tension @xmath16 or the hadronic scale parameter @xmath17 defined through the static potential between quarks @xmath18_{r = r_0}=1.65$].@xcite their values are very close and usually read @xmath19 in lattice calculations there are errors arising from the finite size of the lattice spacing @xmath11 and the finite lattice volume . but for small enough @xmath11 we expect the continuum limit to be approached as @xmath20 with a constant @xmath21.@xcite      the pure gauge spectrum of quenched qcd was investigated initially by morningstar and peardon in an anisotropic lattice.@xcite they used different spacings for the spatial @xmath22 and for the temporal @xmath23 extensions with the ratio @xmath24 . this technique allows to control better the inherent errors induced by the lattice . morningstar and peardon identified 13 glueballs below 4 gev . in order to distinguish single from multiple glueball states , they determined approximately the locations of the two - particles states using the mass estimation of the lowest few particles . in their estimates of these locations they assumed that the two glueballs do not interact and that the threshold for their production is given by the energy @xmath25 where @xmath26 and @xmath27 are the masses of the single glueballs . all states lying below the two - glueball threshold are then single glueball states . these authors pointed out that they can not rule out a single glueball interpretation for higher states . they do not find any hint that their states are torelon pairs . finite volume effects are quite under control . when going from a @xmath28 lattice to a @xmath29 lattice , the fractional changes on the mass @xmath30 are less than a few percent and consistent with zero . the largest effect of these errors occur in the @xmath31 and @xmath32 representations of the octahedral group . they are the main cause of uncertainties in the @xmath33 and @xmath34 glueballs . the proximity of the two glueball thresholds and the finite volume errors on the @xmath35 lead the authors to withhold judgment on whether or not this level is a single glueball .    . final continuum - limit glueball mass estimates @xmath36 . when a unique @xmath37 interpretation for a state can not be made , the other possibilities are indicated in the second column . states whose interpretation requires further study are indicated by a dagger . in column 3 , the first error is the statistical uncertainty from the continuum - limit extrapolation and the second is the estimated uncertainty from the anisotropy . in the final column , the first error comes from the combined uncertainties in @xmath38 , the second from the uncertainty in @xmath39 mev . [ tab : finalmasses ] [ cols="^,^,^,^",options="header " , ]     0.3 cm    the status of glueball observation would become clearer with a combination of data of @xmath40 and @xmath41 machines with large statistics , however since the glueball width into two - photons is small , this would require very high luminosity . it has been pointed out that heavy - ion colliders , due to their large center of mass energy , allow access to the regge region @xmath42 and hence to the production of glueballs in peripheral collisions through photon - photon collisions as well as double pomeron exchange @xcite . distinctive features can be derived from the decays of the glueballs since they are flavour singlets : decays to @xmath43 identify a flavour octet and radiative decays of glueballs are forbidden . however , these arguments have to be taken with care since mixing of a glueball with mesons , having the same quantum numbers , can occur and would dilute any selection rule . an interesting idea recently proposed is that of chiral suppression.@xcite if chiral symmetry breaking in glueball decay is dominated by quark masses , then the coupling of a spin zero glueball to light @xmath44 pairs is chirally suppressed . an interesting consequence is that radiative j/@xmath45 decay becomes a filter for new physics in the @xmath46 channel , since at leading order radiative decays to spin zero light quark mesons are suppressed while radiative decays to @xmath46 glueballs , hybrid , and four quark states are not suppressed . this idea is not without controversy . @xcite    the glueball groundstate @xmath47 would be an ideal state to find since it has many fundamental connotations.@xcite in gluodynamics , as we have seen , the situation that arises from lattice calculations is clear and the masses of the scalar glueballs are large @xmath48  gev . however , when sea quarks are considered no firm conclusion about the scalar spectrum can be drawn . the theoretical calculations based on qcd sum rules and/or low energy theorems lead to contradictory results . its properties , _ i.e. _ , mass , decay channels and widths still differ among the various approaches . experimentally there are too many isoscalar , scalar mesons between 0.6 and 1.75 gev to be explained by the naive quark model alone . from the theoretical point of view there has been a lot of debate about the structure of these states . let us summarize here the different theoretical explanations for the spectrum . jaffe obtains the @xmath49 and the @xmath50 in the bag model as cryptoexotic @xmath51 states.@xcite the p - wave @xmath44 scalar nonet is likely to lie in the region of the other spin - triplet p - wave nonets , with isoscalars roughly between @xmath52 and @xmath53  mev . @xcite dominguez and paver,@xcite bordes , pearrocha and gimnez,@xcite and kisslinger and johnson,@xcite obtain , using low energy theorems and/or sum rule calculations , a low lying and narrow glueball ( mass @xmath54 700  mev and @xmath55 100  mev ) . vento using low energy theorems has also proposed the existence of a low mass ( @xmath56  mev ) and almost sterile scalar glueball.@xcite narison and collaborators,@xcite using sum rules , get a broader ( 200 - 800  mev ) , heavier ( 700 - 1000  mev ) scalar glueball in this range . forkel,@xcite obtains a scalar glueball at 1250  @xmath57  200  mev with a large width ( @xmath58300  mev ) . between @xmath59 and 1750  mev there are three @xmath60 states : @xmath61 , @xmath62 , and @xmath63 . in the analysis of vento @xcite two scalar glueballs appear in this range , an intermediate mass glueball ( @xmath64 mev ) and another one in the upper range ( @xmath65 mev ) . the narrow state at 1500  mev , discovered in @xmath66 annihilation by crystal barrel ( cb ) is considered by amsler and close a good candidate for the glueball groundstate@xcite while chanowitz considers the @xmath63 as the scalar groundstate@xcite . finally , that the scalar glueball is shared by both resonances is also contemplated.@xcite relativistic heavy - ion collisions might be a tool to produce glueballs . it is conceivable that glueball production becomes a dominant part in central nucleon collisions . two scenarios will be analyzed :    * * quark gluon plasma phase * : one expects that at some point above a certain critical temperature a plasma of quarks and gluons , named quark - gluon plasma ( qgp ) is formed.@xcite this phase is characterized by a large amount of thermal gluons . as the qgp cools down the gluons can form singlet configurations via the color interaction . * * strong coulomb phase * : a recent formulation of the dynamics in the region above the transition temperature @xmath67 , based on a description of recent experiments in ultra relativistic heavy ion collisions,@xcite states that , despite de - confinement , the color coulomb interaction between the constituents is strong and a large number of binary ( even color ) bound states , with a specific mass pattern , are formed.@xcite this phase we call strong coulomb phase ( scp ) . the qgp phase occurs at a much higher temperature @xmath68 when the bound states dissolve . let us describe some attempts to find signatures arising from qgp . the basic idea is that during the hadronization process in this gluon rich environment the gluons combine due to the strong force into glueballs . as the qgp cools further and transforms into hadronic matter , these glueballs decays into conventional hadrons giving rise to signatures of their existence . it has been claimed that the existence of glueballs alters the @xmath69 ratio in the final state.@xcite    it is conceivable that glueball production becomes dominant in central nuclear collisions since the existence of qgp provides a gluon rich environment especially at high energy density . in this scenario the lowest mass glueballs are copiously produced . particular decay modes @xmath70 and @xmath71 have been investigated@xcite and enhancements associated with possible glueball production observed . search strategies , including dipion production have been also proposed.@xcite    estimates within thermal models for the multiplicities of scalar glueballs in central au - au collisions at present and future experimentally available energies , _ i.e. _ from ags to lhc , have been presented.@xcite for the experimental identification of glueballs one can use the decay modes @xmath72 , @xmath73 and @xmath74 . despite of small branching ratio , the @xmath75 channel has the important advantage that photons have practically no rescattering in the hadronic medium . this analysis leads to maximal multiplicities for the glueball of @xmath76 of the @xmath15 meson multiplicity . even larger yields are expected in the case of explosive hadronization of the quark gluon plasma.@xcite    let us now turn to the second scenario . from its inception two decades ago , the high - t phase of qcd commonly known as the quark - gluon plasma,@xcite was described as a weakly interacting gas of `` quasiparticles''(quarks and gluons ) . indeed , at very high temperatures asymptotic freedom causes the electric coupling to be small and the qgp to be weakly interacting . at intermediate temperatures of few times the critical temperature @xmath77 of immediate relevance to current experiments , there is new and growing evidence that the qgp is not weakly coupled . in this region qcd seems to be close to _ a strongly coupled coulomb regime _ , with an effective coupling constant @xmath78 0.5 - 1 and multiple _ bound states _ of quasiparticles.@xcite this phase we call strong coulomb phase . this description is not universally accepted since some lattice calculations do not to find bound states above the transition.@xcite also , one must be aware that other explanations of the data have been presented.@xcite in order to clarify these issues it is interesting to find clear experimental observables that would shed some light into the discussion . -0.0 cm    0.5 cm    0.5 cm    kochelev and min consider the properties of scalar and pseudoscalar glueballs in scp.@xcite in an effective lagrangian approach , based on the low - energy qcd theorems , they find out that scalar glueball remains massive above deconfinement temperature ( see fig . [ glueball ] ) . at the same time , pseudoscalar glueball changes its properties in qgp in a drastic way . indeed , this glueball becomes massless at @xmath79 and therefore it can contribute strongly to , the bulk properties of the plasma ( see fig . [ glueball ] ) . they demonstrate that the disappearance of pseudoscalar glueball mass above the deconfinement temperature and its strong coupling to gluons gives the rise to the sign change of the gluon condensate in the pure @xmath80 gauge theory as observed in the lattice calculations at @xmath81 ( see fig . [ glueball]).@xcite the strong nonperturbative coupling of the glueball to the gluons leads to the conjecture that one might expect that the role of very light pseudoscalar glueball in qgp must be quite similar to the role played by the massless pion in nuclear matter below deconfinement temperature .    in scp despite de - confinement the color coulomb interaction between the constituents is strong and a large number of binary ( even color ) bound states , with a specific mass pattern , are formed.@xcite with this input , the scenario envisaged by vento for gluodynamics , _ i.e. _ the theory with only gluons goes as follows.@xcite the strong coulomb phase is crowded with gluon bound states and the lightest is the scalar glueball , labelled @xmath82 . as one moves towards the dilution limit , the binding energy of these states decreases , the gluon mass increases , and therefore the color and singlet bound states increase their mass softly until the gluons are liberated forming a liquid.@xcite however , as the system cools towards the confining phase , color and singlet states decay into the conventional low lying glueballs , in particular @xmath82 . thus the number of @xmath82 s becomes large . this reasoning generalizes to qcd since in the scp the multiplicity of glueball channels is larger than in the confined phase . the ratio of glueball to meson channels goes from 1 to 8 below the phase transition to 1 to 2 above.@xcite thus the number of scalar glueballs is much larger in scp than in the cold world .    as the fireball cools a  large number " of gluonic bound states decay by gluon emission into @xmath82 s . the emitted gluons form new bound states of lower mass due to the strong color coulomb interaction . as we approach the confinement region the mass of the color bound states increases and it pays off to make multiparticle color singlet states , which decay by rearrangement into ordinary color singlet states . since the coupling is strong and the phase space is large , these processes take place rapidly . thus in no time , close to the phase transition temperature @xmath67 , a large number of scalar glueballs populate the hadronic liquid . 0.5 cm    0.5 cm    let us now analyze the experimental signal of these phenomenon . if we assume that the @xmath83 is the o(4 ) partner of the @xmath84 in the chiral symmetry realization of qcd , its mass decreases when approaching the phase transition , becoming degenerate with the pion at @xmath67 ( see fig . [ scp ] ) . beyond @xmath67 , in the scp , chiral symmetry is restored , and @xmath84 and @xmath83 remain degenerate for @xmath85 . thus in the scp the @xmath83 can only decay in @xmath75 for obvious kinematical reasons . the glueball @xmath82 does not vary its mass in this region appreciably . thus even before we reach @xmath67 , the mixing between @xmath82 and @xmath83 disappears ( see fig . [ scp ] ) and @xmath82 becomes stable around @xmath67 . however , in the scp the mass of the @xmath83 increases and in a certain region of @xmath86 it again becomes degenerate with @xmath82 and mixing is restored . thus the physical @xmath82 is able to decay , once the @xmath83 component is attained , only to @xmath75 . the enhancement in the number of @xmath82s with respect to the hadronic phase arises because of the larger population of glueballs in the scp , as described above , and because these particles are stable in the medium against the dominating hadronic decays ( see fig . [ scp ] ) . thus a clear signal for the existence of a low mass scalar glueball and a confirmation of the scp scenario would be two @xmath75 peaks corresponding to @xmath82 and the @xmath83-meson as shown in fig . [ fits ] . 0.5 cm    the investigation on jets in the relativistic heavy ion collisions at rhic provides a deep insight into the properties of the quark - gluon plasma.@xcite one of the important rhic discoveries is the jet quenching phenomenon coming from the partonic energy loss in qgp . in the conventional approach to the jet quenching the perturbative ( pqcd ) type of energy loss is taken into account by the channels , elastic and radiative , of one - gluon exchange between the jet and the massless gluons and quarks.@xcite however , the large quark - gluon rapidity density @xmath87 which is needed to describe the rhic jet quenching data within this approach , seems to be in contradiction with the restriction @xmath88 coming from the measured final entropy density @xmath89.@xcite furthermore , the lattice calculations show that even at very high temperature gluons and quarks still interact strongly in qgp . @xcite    -0.0 cm    0.5 cm    as seen above , it has been suggested that the glueballs , the bound states of gluons , can exist above deconfinement temperature and may play an important role in the dynamic of strongly interacting qgp.@xcite in particular , it has been suggested that a very light pseudoscalar glueball can exist in qgp and might be responsible for the residual strong interaction between gluons.@xcite the lattice results showing a change of sign of the gluon condensate@xcite and a small value of the topological susceptibility above @xmath77 can be explained in the glueball picture as well . furthermore , one expects that the suppression of the mixing between glueballs and quarkonium states in the qgp leads to a smaller width for the former as compared to the vacuum.@xcite this property opens the possibility for a clear separation of the glueball and the quark states in heavy ion collisions . such separation is rather difficult in other hadron reactions due to existence of strong glueball - quarkonium mixing in the vacuum . min and kochelev made an estimate of the energy loss induced by interaction of an energetic parton , which was produced in the hard scattering of two heavy ion s partons , with glueballs in the hot quark - gluon plasma.@xcite they showed that such contribution leads to a significant energy loss as can be seen from fig . in particular , for the gluon jet such contribution is about a few gev / fm and approximately twice larger than the perturbative elastic loss.@xcite it should be pointed out that more than one half of contribution to the gluon energy loss comes from interaction of gluon with the light pseudoscalar glueball in qgp . thus in spite of the fact that for the quark jet the glueball contribution is smaller than perturbative elastic loss , it can not be neglected in comparison with latter one . the existence of such light bound state of gluons above @xmath67 is crucial for the understanding of the large observed partonic energy loss in qgp . therefore , not only pqcd type of energy loss but also glueball - induced loss , arising from existence of scalar and pseudoscalar glueballs in qgp , are important for the understanding of the rhic results such as the jet quenching . it is well known that in the many high energy reactions with small momentum transfer the exchange by the highest - lying regge trajectory , called as a soft pomeron , gives the dominant contribution . this exchange carries vacuum quantum numbers and has very peculiar properties in comparison with the usual regge pole trajectories as the @xmath90 , @xmath84 , _ etc_. from the analysis of the varions cross sections it follows that the pomeron trajectory has a linear behavior @xmath91 the pomeron does not seem to be related to the usual mesons since the latter have usually lower intercepts and very different slopes . there has been a long - standing speculation that the physical particles on the trajectory might be glueballs . meyer and teper investigated the pure gauge spectrum on a lattice to check its compatibility with a linear trajectory.@xcite they calculated the lightest states @xmath92 since the trajectory has an even signature . their masses ( within errors bars ) are consistent with the pomeron trajectory . these studies of quarkless qcd helps to understand the close relation between the pomeron and the pure glue states , but it is worth mentioning that the physical particles should probably mix with quarkonia . the linearity of the trajectory , supported by these arguments , favors flux tube and stringy pictures as discussed in the constituent model section , at least for ( even)@xmath93 glueballs . most of our review has been dedicated to the study of pure glueball states . we have assumed in our discussion that glueballs are physical objects , which is fine for the purpose of the developments thus far . but in order to establish contact with reality we have to discuss in some detail mixing , an idea which has been mentioned in passing on several occasions . we first discuss the mathematics of mixing in a two state model and then apply the reasoning to the physical reality in the isosinglet scalar sector . mixing arises when the total hamiltonian is not diagonal in the glueball and @xmath95 fock space . in this case the physical states come out as superpositions of glueballs and @xmath96 states . let us assume that we have a pure glueball , we label @xmath82 , and a @xmath95 state , we label @xmath83 , which have the same quantum numbers and , for simplicity , also the same mass @xmath97 , in a certain approximation . we next relax this approximation and the hamiltonian in the reduced @xmath82 and @xmath83 fock space becomes @xmath98 the diagonal basis of this hamiltonian can be presented as , @xmath99 where the tilde labels the physical particles and @xmath100 is the mixing angle . the masses of the physical particles become @xmath101 where @xmath102 and @xmath103    in fig . [ 2statemixmass ] we represent the masses of the physical states as a function of the mixing angle @xmath100 . the curves separate possible mass regions . the curves show that the two state mixing scenario for positive @xmath104 leads to a  light " glueball and heavier meson . the opposite result can be obtained with a negative @xmath104 . the procedure can be generalized to more states and to different initial masses as we next see . this mechanism has been used to try to disentangle the complex isoscalar sector . vento has shown that a low mass , almost invisible isosinglet scalar glueball , could hide in the low energy spectrum due to small mixing with the @xmath49 , and pointed out the possible existence of other glueballs components among the remaining isosinglet scalars @xmath61 , @xmath62 and @xmath63.@xcite    close _ et al . _ have discussed the content of the the isosinglet scalar mesons @xmath61 , @xmath62 and @xmath63 in an attempt to discover which of them is dominantly a scalar glueball.@xcite these authors have suggested that @xmath62 is primarily a scalar glueball , due partly to the fact that @xmath62 , discovered in @xmath105 annihilation at lear , has decays to @xmath106 and @xmath107 which are relatively large compared to that of @xmath108@xcite and that the earlier quenched lattice calculations@xcite predict the scalar glueball mass to be around @xmath109 mev . furthermore , because of the small production of @xmath108 in @xmath63 decay compared to that of @xmath110 , they claim that the @xmath63 is primarily @xmath111 dominated . in contrast , the smaller production rate of @xmath110 relative to @xmath108 in @xmath61 decay leads to the conjecture that @xmath61 is governed by the non - strange light quark content . based on these observations , they have proposed a flavor - mixing scheme to consider the glueball and @xmath112 mixing in the neutral scalar mesons @xmath63 , @xmath62 and @xmath61.@xcite fits to the measured scalar meson masses and their branching ratios of strong decays have been performed in several@xcite leading to a mixing matrix of the form @xmath113 where @xmath114 and @xmath115 denote the quarkonium states @xmath116 and @xmath117 , and @xmath118 denotes the pure scalar glueball state . thus for close _ the @xmath62 is composed primarily of a scalar glueball . another analysis has been carried out by cheng _ et al._.@xcite two lattice results are employed as the starting point ; one is the approximate su(3 ) symmetry in the scalar sector above 1 gev for the connected insertion part without @xmath112 annihilation,@xcite and the other is the scalar glueball mass at 1710 mev in the quenched approximation.@xcite in the su(3 ) symmetry limit , @xmath62 becomes a pure su(3 ) octet and is degenerate with @xmath119 , while @xmath61 is mainly an su(3 ) singlet with a slight mixing with the scalar glueball which is the primary component of @xmath63 . these features remain essentially unchanged even when su(3 ) breaking is taken into account . the observed enhancement of @xmath120 production over @xmath121 in hadronic j/@xmath45 decays and the copious @xmath63 production in radiative j/@xmath45 decays lend further support to the prominent glueball nature of @xmath63 . furthermore , chiral suppression@xcite is advocated to obtain the following mixing matrix , @xmath122    therefore for cheng _ _ it is the @xmath63 is the particle composed mostly of a glueball state . thus a definitive conclusion on this problem is still lacking .    finally we would like to mention that the quarkonium - glueball mixing gives also a strong influence to the properties of the lowest mass pseudoscalar glueball.@xcite the leitmotif of this review has been that the study of glueballs , states where the gauge field plays an important dynamical role , is important to understand the nonperturbative behavior of qcd . this study requires precise and abundant experimental input , which despite a lot of effort has not achieved a level of understanding that allows a unique theoretical interpretation . the lack of clarity arises from the fact that the theoretical developments are not able to determine in a well defined manner how the production and decay properties of glueballs are distinct from those of conventional mesons . but the future is bright for several reasons . new energy domains will be studied by besiii and in the future by @xmath123anda , and therefore the possibility of producing oddballs , glueballs with exotic quantum numbers will open . moreover , at low energy , both crystal barrel and in the future gluex will produce light mesons with a level of statistics , and precision for analysis , that will be able to separate exotic from non exotic behaviors and quark model nonets from particles extraneous to them . in this process the new theoretical developments will be crucial to guide and/or interpret the results . in this review we have discussed three major approaches , lattice qcd , qcd based constituent models and qcd sum rules , with a short excursion into the ads / qcd formalism , which at present leads ultimately to a constituent model type of description .    due to the lack of observable states , the lattice qcd results have played the role of experimental data . lattice qcd is a powerful technique , especially for the determination of masses . the spectrum of the pure gluodynamics ( the pure gauge theory ) , equivalent to the so called quenched approximation of lattice qcd , is well established and the calculational errors are under control . however the various treatments of quark loops ( unquenched lattice qcd ) and the effects of the mixing of glueballs with mesons are still a matter of debate and no firm conclusions can be drawn for the real world . moreover , important features of the study would be the determination of the decay properties of the various glueball candidates , and lattice qcd being a theory described in euclidean space - time does not possess the capability of describing asymptotic states and therefore of determining decay properties . however , a full lattice qcd calculation with unlimited and precise calculational power should produce the experimental spectrum , if as we strongly believe qcd is the theory of the hadronic interactions , and moreover , if one can extend lattice qcd into minkowski space - time one would be able to determine the decay properties . given the lack of experimental and detailed theoretical knowledge , models turn out to be an interesting laboratory to test ideas of the various perturbative and nonperturbative mechanisms in the theory . we have discussed in some detail constituent gluon models for glueballs from different perspectives . most of the pioneer work considered glueballs as strongly bound states of two ( or three ) heavy spin-1 gluons . the comparison with lattice qcd shows that the spectrum obtained by these approaches does not correspond to lattice qcd . however , if one implements a formalism dealing with only transverse gluons , the spurious states ( induced by the gluonic longitudinal components ) disappear and the hierarchy in the spectrum of lattice qcd is recovered . these improvements have been developed up to now only in the two - gluon sectors and the generalization to negative @xmath124-parity is still lacking . it remains for the future to find out which constituent glueball model confirms the lattice qcd spectrum for the higher states . the dynamics in these models is described , in general , by a linear plus coulomb potentials . this dynamics , however , does not remove the degeneracy of the scalar and pseudoscalar glueballs . the candidate instanton induced interaction , which arises naturally in qcd and which has very particular properties under spin - parity , is the natural candidate to do the job . in the quenched approximation , the instanton induced interaction is attractive in the scalar and repulsive in the pseudoscalar channels , and equal in magnitude . this is the precise behavior needed to lift the degeneracy in the constituent models . the qcd sum rules offer the possibility to describe mixing and sea quarks effects in glueball states . the method is rather technical and requires many ingredients ( condensates , instantons , topological charge screening , ... ) . this complexity has led to many different calculations , with different results for the lowest lying states . we refer the reader to the literature for the various attempts to describe the low mass glueball spectrum . in here , we present the general formalism and some details of a specific calculation , in which the masses are obtained from pure gluonic currents , however , sea quarks effects are included ( _ via _ the quark condensate ) . glueballs should be produced preferably in gluon - rich processes . we described briefly the main examples : j/@xmath45 decays , central production @xmath125 annihilation,@xmath126 fusion and photoproduction . the realization of a new generation of experiments , besiii , @xmath123anda and gluex , provide hope for new exciting developments in this field . the ultra - relativistic heavy ion experiments is another experimental scenario where one expects that glueballs might play a role . it is clear that the transition to a different phase of confinement , be it the strong coulomb phase or the quark gluon plasma , precludes that the confinement properties of the bound systems will change and with it their physical behavior . there is no consensus on the glueball properties above the critical temperature and this has led to the description of several scenarios which can be very interesting from the point of view of observation . rhic has produced large amount of date at high temperature , as will very soon do alice . on the other hand fair will probe the high density region . given these circumstances it is clear that the aim is to performed realistic analysis which are are able to disentangle the right from the wrong ideas . finally , the strong expected mixing between glueballs and quark states leads to a broadening of the possible glueball states which does not simplify their isolation and their theoretical description . the wishful sharp resonances which would confer the glueball spectra the beauty , richness and simplicity of the conventional baryonic and mesonic spectra are lacking . we hope that in the future and at higher energies the situation changes and we are able to have isolated exotic states . it is , however , important to stress , that in any case gluebals are a beautiful and unique consequence of qcd . the study of glueballs is intimately related to the quantitative understanding of confinement in qcd , since understanding confinement requires an understanding of the soft gluonic field responsible for binding the hadrons and the structure of the qcd vacuum . we have emphasized that this has been the goal of the various studies , _ i.e. _ to get a clearer picture of how qcd behaves at relatively low momentum and how this behavior changes as the temperature or the density increase . our review shows that we have learned much but we have yet not achieved the goal . we foresee a bright future in this respect since a new experimental era of the study of qcd in many fronts is opening up . to find a description , which is able to describe all the various phenomena in a unique framework , is our task , and when achieved will imply that we finally understand qcd . the venture is even more exciting since maybe ads gravity might come to our help , a possibility unthinkable a few years back . we would like to thank philippe de forcrand , alexander dorokhov , hilmar forkel , sergo gerasimov and dong - 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glueballs are particles whose valence degrees of freedom are gluons and therefore in their description the gauge field plays a dominant role . 
 we review recent results in the physics of glueballs with the aim set on phenomenology and discuss the possibility of finding them in conventional hadronic experiments and in the quark gluon plasma . in order to describe their properties 
 we resort to a variety of theoretical treatments which include , lattice qcd , constituent models , ads / qcd methods , and qcd sum rules . 
 the review is supposed to be an informed guide to the literature . 
 therefore , we do not discuss in detail technical developments but refer the reader to the appropriate references .