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in hadron - hadron scattering , interactions are classified by the characteristics of the final states . in elastic scattering , both hadrons emerge unscathed and no other particles are produced . in diffractive dissociation , the energy transfer between the two interacting hadrons remains small , but one ( single dissociation ) or both ( double dissociation ) hadrons dissociate into multi - particle final states , preserving the quantum numbers of the associated initial hadron . the remaining configurations correspond to inelastic interactions . the most difficult conceptual aspect of diffractive scattering is to provide a unique and concise definition . this will not be attempted here and diffraction will be understood as an interaction between projectile and target that generates a large rapidity gap between the respective final states , which is not exponentially suppressed . diffractive interactions are mediated by the exchange of a colorless object , with quantum numbers of the vacuum . this definition fits very well the framework of soft interactions , where diffractive scattering is mediated by the exchange of the universal pomeron trajectory ( @xmath0 ) , introduced by gribov  @xcite . ingelman and schlein  @xcite proposed to use diffractive scattering in the presence of a large scale to establish the partonic content of the pomeron .    in qcd , the candidate for vacuum exchange with properties similar to the soft pomeron is two gluon exchange  @xcite . as a result of interactions between the two gluons , a ladder structure develops . in perturbative qcd ( pqcd ) , the properties of this ladder depend on the energy and scales involved in the interaction , implying its non - universal character . each of the approaches mentioned above leads to definite predictions , which can be tested in high energy diffractive interactions in the presence of a hard scale . this has been pursued in @xmath1 scattering at hera and in @xmath2 scattering at the tevatron . the purpose of this talk is to summarize the recently achieved progress . the variables used to analyze diffractive scattering will be introduced for deep inelastic @xmath1 scattering ( dis ) . since dis is perceived as a two - step process , in which the incoming lepton emits a photon which then interacts with the proton target , the relevant variables can be readily generalized to @xmath2 interactions .     interactions . _ _ ]    a diagram for diffractive scattering in dis , where the diffracted state is separated from the scattered proton by a large rapidity gap ( lrg ) , is presented in figure  [ fig : dis - diag ] and all the relevant four vectors are defined therein . the usual dis variables are the negative of the mass squared of the virtual photon , @xmath3 , the square of the center of mass energy of the @xmath4 system , @xmath5 , the bjorken scaling variable , @xmath6 , which in the quark parton model constitutes the fraction of the proton momentum carried by the interacting quark , and the inelasticity , @xmath7 . in addition to the usual dis variables , the variables used to described the diffractive final state are , @xmath8 @xmath9 is the fractional proton momentum which participates in the interaction with @xmath10 . it is sometimes denoted by @xmath11 . @xmath12 is the equivalent of bjorken @xmath13 but relative to the exchanged state . @xmath14 is the invariant mass of the hadronic final state recoiling against the leading proton , @xmath15 . the approximate relations hold for small values of the four - momentum transfer squared @xmath16 and large @xmath17 , typical of high energy diffraction . to describe diffractive dis , it is customary to choose the variables @xmath9 and @xmath16 in addition to the usual @xmath13 and @xmath18 in the cross section formula . the diffractive contribution to @xmath19 is denoted by @xmath20 and the corresponding differential contribution , integrated over @xmath16 , is @xmath21 the three - fold differential cross section for @xmath1 scattering can be written as @xmath22   \sigma_r^{d(3)}(x , q^2,{x_{{{\scriptscriptstyle { i\!\!p } } } } } ) \ , ,   \label{eq : f2d4}\ ] ] where @xmath23 @xmath24 stands for the diffractive longitudinal structure function , which may not be small . the structure function @xmath19 is related to the absorption cross section of a virtual photon by the proton , @xmath25 . for diffractive scattering , in the limit of high @xmath17 ( low @xmath13 ) , @xmath26 this relation allows predictions for diffractive scattering in dis based on regge phenomenology applied to @xmath4 scattering . in fact many of the questions that are addressed in analyzing diffractive scattering are inspired by regge phenomenology as established in soft hadron - hadron interactions . the scattering of two hadrons , @xmath27 and @xmath28 , at squared center of mass energy @xmath29 , is described by the exchange of the universal @xmath0 trajectory parameterized as @xmath30 . the @xmath0 trajectory determines the @xmath31 dependence of the total cross section , @xmath32 . the ratio of elastic and diffractive to total cross sections , is expected to rise like @xmath33 . a steep and universal @xmath9 dependence of the diffractive cross section is expected , @xmath34 . values of @xmath35  @xcite and @xmath36  @xcite were derived based on total hadron - proton interaction cross sections and elastic proton - proton data . recently the @xmath0 intercept has been reevaluated  @xcite leading to a value of @xmath37 . the positive value of @xmath38 implies that the slope of the @xmath16 distribution is increasing with @xmath39 . this fact , borne out by the hadron - hadron and photoproduction data ( for a review and new data see  @xcite ) , is known as shrinkage of the @xmath16 distribution . it is due to the fact that @xmath40 and has been explained by gribov  @xcite as diffusion of particles in the exchange towards low transverse momenta , @xmath41 , with @xmath42 ( see also  @xcite ) . qcd factorization for the diffractive structure function of the proton , @xmath43 , is expected to hold  @xcite , while it can not be proven for hadron - hadron interactions  @xcite . @xmath43 is decomposed into diffractive parton distributions , @xmath44 , in a way similar to the inclusive @xmath19 , @xmath45 where @xmath46 is the universal structure function for dis on parton @xmath47 , @xmath48 is the factorization scale at which @xmath44 are probed and @xmath49 is the fraction of momentum of the proton carried by the diffractive parton @xmath47 . diffractive partons are to be understood as those which lead to a diffractive final state . the dglap evolution equation applies in the same way as for the inclusive case . for a fixed value of @xmath9 , the evolution in @xmath13 and @xmath18 is equivalent to the evolution in @xmath12 and @xmath18 . if , following ingelman and schlein  @xcite , one further assumes the validity of regge factorization , @xmath43 may be decomposed into a universal @xmath0 flux and the structure function of the @xmath0 , @xmath50 where the normalization of either of the two components is arbitrary . it implies that the @xmath9 and @xmath16 dependence of the diffractive cross section is universal , independent of @xmath18 and @xmath12 , and given by @xmath51 one of the expectations which is subject to experimental tests . the mechanism for producing lrg is assumed to be present at some scale and the evolution formalism allows to probe the underlying partonic structure . the latter depends on the coupling of quarks and gluons to the pomeron . at hera the diffractive candidate events are selected either by requiring a large rapidity gap  @xcite , or by requiring a leading proton  @xcite . the various analyzes differ in the way the non - diffractive contributions are treated and in the way the proton dissociative events are subtracted . the comparison between the various measurements is shown in figure  [ fig : f2dcomp ] .    the @xmath9 dependence of @xmath52 , in the region of @xmath53 , is expected to be dominated by @xmath0 exchange . the corresponding values of @xmath54 are shown as a function of @xmath18 in figure  [ fig : f2dcomp ] . a dependence of @xmath55 on @xmath18 can not be excluded , however the errors are large enough , so that a constant value of @xmath56 fits the data . however averaged over the whole @xmath18 range the value of @xmath55 is definitely larger than the intercept of the soft pomeron . in addition , the diffractive @xmath57 value is only half of that for inclusive @xmath19 measurements , also shown in the figure . this means that the ratio of diffractive to total @xmath4 cross sections is constant with @xmath17 . those are indications that the connection of diffractive dis to the simple soft @xmath0 picture is not straight forward . the h1 measurements of @xmath52  @xcite , which cover by far the largest phase space , have been used to perform a qcd evolution fit to extract diffractive parton distribution functions ( dpdf ) . the results of the fit are shown in figure  [ fig : dpdf ] . a very good description of the data is obtained , provided the parton distributions are dominated by gluons , which carry about 80% of the momentum of partons leading to diffractive events . this latest extraction of dpdfs by h1 is called h1 2002 dpdfs . qcd factorization can be tested in high transverse momenta , @xmath58 , jet production in @xmath4 , @xmath59 and @xmath2 diffractive production . if factorization holds , the cross section for production of jets  @xcite and charm  @xcite in @xmath4 should be well reproduced by nlo calculations with dpdfs , extracted from structure function measurements . this is indeed the case as demonstrated in figure  [ fig : jetscc ] . qcd factorization breaking is observed in hard diffractive scattering in @xmath2 . measurements  @xcite of two jet production accompanied by the presence of a leading anti - proton have been used to extract the effective diffractive structure function for two jet production @xmath60 , which can then be compared with the expectations from dpdf extracted at hera . as shown in figure  [ fig : fnaljj ] , even for the h1 2002 dpdfs , where the abundance of gluons is lesser compared to earlier dpdfs  @xcite , the expectations are by about a factor 10 above the measurements . interactions as a function of @xmath12 , compared to expectations of different sets of dpdfs : left , for single pomeron exchange , right , double pomeron exchange.__,width=207 ]    it should be stressed however , that the range of @xmath11 ( that is @xmath9 ) covered by the @xmath2 measurements is beyond the range probed by the h1 data , from which the dpdfs originate . in extrapolating the h1 parameterization into this high @xmath11 region , the reggeon contribution is estimated to be of the order of 30 to 40% . a lesser qcd factorization breaking is observed for diffractive dijet production in @xmath2 events in which both baryons remain unscathed - the so called double pomeron exchange ( dpe ) process  @xcite . compared to expectations , the rate of dijet production is only by about factor two less abundant . this is shown in figure  [ fig : fnaljj ] . effects of factorization breaking are also expected for quasi - real @xmath59 interactions , due to the presence of the resolved photon component . the measurements of diffractive dijets by the h1 experiment have been compared to nlo calculations  @xcite based on the h1 2002 dpdfs . , for the diffractive @xmath61 reaction , compared to nlo calculations without ( @xmath62 ) or with ( @xmath63 ) suppression of the resolved photon contribution . @xmath64 denotes the diffracted system accompanying the two jets , separated by a large rapidity gap from the proton or its dissociative state @xmath65 . _ , width=207 ]    as shown in figure  [ fig : xgam ] , a good agreement with data is obtained if the resolved photon contribution is suppressed relative to the direct contribution . the factor 0.34 that multiplies the resolved component in the calculation was motivated by the recent work of kaidalov et al . kaidalov et al . @xcite investigated what fraction of the gluon distribution in the proton leads to diffractive final states . the ratio of diffractive to inclusive dijet production cross sections as a function of @xmath13 of the gluon , for different hard scattering scales and for the h1 2002 dpdfs is presented in figure  [ fig : xgam2 ] .     of the gluon for different scales of the hard scattering , for the h1 2002 dpdfs . also shown is the unitarity limit , called pumplin bound . _ _ ]    this ratio should be smaller than 0.5  @xcite , while for scales @xmath66 this limit is exceeded for @xmath67 . this indicates that unitarity effects may already be present in diffractive scattering and may explain why the rise of diffractive scattering cross section with @xmath17 is slower than expected ( see section  [ sec : incl ] ) . the dynamics behind diffractive dis can be easier understood if the process is viewed in the rest frame of the proton . the virtual photon develops a partonic fluctuations , whose lifetime is @xmath68  @xcite . at the small @xmath13 typical of hera , where @xmath69 , it is the partonic state rather than the photon that scatters off the proton . if the scattering is elastic , the final state will have the features of diffraction . the fluctuations of the @xmath10 are described by the wave functions of the transversely and longitudinally polarized @xmath10 which are known from perturbative qcd . small and large partonic configurations of the photon fluctuation are present . for large configurations non - perturbative effects dominate in the interaction and the treatment of this contribution is subject to modeling . for a small configuration of partons ( large relative @xmath41 ) the total interaction cross section of the created color dipole on a proton target is given by  @xcite @xmath70 where @xmath71 is the transverse size of the color dipole and @xmath72 is the scale at which the gluon distribution @xmath73 of the proton is probed . the corresponding elastic cross section is obtained from the optical theorem . in this picture , the gluon dominance in diffraction results from the dynamics of perturbative qcd ( see equation  ( [ eq : qqgp ] ) ) . models of diffraction that follow this approach are quite successful in describing both the inclusive @xmath19 and the diffractive @xmath20 measurements , where the former are used to parameterize the dipole - proton cross section . an example taken from  @xcite is shown in figure  [ fig : dipole ] . measurements.__,width=321 ]    it is interesting to note that the two models  @xcite which predict a relatively mild increase of @xmath52 with decreasing @xmath9 are the ones which explicitly include effects of unitarization through saturation of cross sections for large size dipoles . the saturation scale is increasing with increasing @xmath17 . the dynamical origin of this form of saturation can be derived from a new form of qcd matter , called color gluon condensate  @xcite . in this approach , the data suggest that the saturation scale @xmath74 for @xmath75 . the presence of small size @xmath76 configurations in the photon can be tested in exclusive vector meson ( vm ) production as well as for deeply inelastic compton scattering . at high energy ( low @xmath13 ) and in the presence of a large scale ( large @xmath18 or heavy flavor ) , these reactions are expected to be driven by two - gluon exchange .    a closer look at the theory of exclusive processes in qcd shows that the two partons taking part in the exchange do not carry the same fraction of the proton momentum . that makes these processes sensitive to correlations between partons , which are encoded in the so - called generalized parton distributions , gpds  @xcite . these new constructs relate in various limits to the parton distributions , form factors and orbital angular momentum distributions . the motivation behind studies of exclusive processes is to establish the region of validity of pqcd expectations and ultimately to pursue a full mapping of the proton structure , which can not be achieved in inclusive measurements . the cross section for the exclusive processes is expected to rise with @xmath17 , with the rate of growth increasing with the value of the hard scale . a compilation of logarithmic derivatives @xmath77 , for @xmath78  @xcite , @xmath79  @xcite and @xmath80  @xcite exclusive production , as a function of the scale defined as @xmath81 , where @xmath82 is the mass of the vm , is presented in figure  [ fig : delta ] .    with decreasing transverse size of the dipole , the @xmath16 distribution is expected to become universal , independent of the scale and of the vm . the exponential slope of the @xmath16 distribution , @xmath28 , reflects then the size of the proton . a compilation  @xcite of measured @xmath28 values is presented in figure  [ fig : delta ] . around @xmath83 of about @xmath84 indeed the @xmath28 values become universal . another important manifestation of the perturbative nature of exclusive processes , related to the universality of the @xmath16 distribution , is that the slope , @xmath38 , of the corresponding regge trajectory should become small . the parameters of the effective regge trajectory can be determined in the study of the @xmath17 dependence of the differential cross section for exclusive processes at fixed @xmath16 . the results obtained for exclusive vector meson production  @xcite are compiled in figure  [ fig : apom ] . ( dots ) and @xmath38 ( open squares ) values , extracted in exclusive vm production , as a function of @xmath83 . _ , width=207 ]      the deeply virtual compton scattering ( dvcs ) process , @xmath85 , has been advocated as one of the exclusive processes for which theoretical calculations are free of uncertainties due to hadronic wave function uncertainties  @xcite . in addition , the interference of the dvcs and qed bethe - heitler amplitudes for prompt @xmath86 production is proportional to the real part of the qcd amplitude , which in turn is sensitive to gpds . the extraction of the dvcs cross section in @xmath1 scattering has been performed by the h1  @xcite and zeus  @xcite experiments . a clear rise of the dvcs cross section with @xmath17 has been observed  @xcite as shown in figure  [ fig : dvcs ] .     as a function of @xmath17 ( left ) for fixed @xmath18 and as a function of @xmath18 for fixed @xmath17 ( right).__,width=283 ]    a comparison of the dvcs cross section dependence on @xmath17 and @xmath18 with two approaches , one based on the dipole model  @xcite and the other on nlo evolution of gpds with postulated initial conditions  @xcite , is shown in figure  [ fig : dvcs ] . a good agreement with data is obtained . diffractive production of vm or prompt @xmath86 at large values of @xmath16 accompanied by proton dissociation , form another class of interactions which are of interest for understanding the high energy regime of pqcd . the large value of @xmath16 accompanied by a large rapidity gap , suggests the applicability of the leading logarithmic bfkl dynamics  @xcite .    the @xmath16 distribution for the process @xmath87 @xcite , where the lrg separates the photon from the dissociated proton state @xmath65 is shown in figure  [ fig : largetp ] .     at the proton vertex in the process @xmath88 . the lines represent theoretical expectations based on bfkl , as described in the figure . right : @xmath17 dependence of @xmath89 in bins of @xmath16 measured at the proton vertex . the lines represent theoretical expectations based on the bfkl or dglap dynamics , as described in the figure . _ ]    the @xmath16 distribution is well represented by calculations based on leading logarithmic bfkl approximation  @xcite . the bfkl approach also describes well the @xmath17 dependence of the cross section for exclusive @xmath80 production at large @xmath16 , while expectations based on the leading logarithmic dglap dynamics fail to describe the observed dependence for @xmath90 . this is shown in figure  [ fig : largetp ] . this is yet another indication that the dglap dynamics , which is successfully used to describe the measurements of @xmath19 at hera  @xcite , may not be sufficient to describe all the features of dis at high energy . the exclusive diffractive production of the higgs boson has been proposed  @xcite as a potential background - free method to search for the light higgs at lhc . a process similar to exclusive higgs production is the exclusive @xmath91 production , for which at the tevatron the cross section is predicted to be about @xmath92  @xcite . the diagram corresponding to the proposed diffractive process is shown in figure  [ fig : higgs ] . production in @xmath2 scattering . right : invariant mass distribution of the candidate exclusive @xmath93 system . _ , width=132 ]     production in @xmath2 scattering . right : invariant mass distribution of the candidate exclusive @xmath93 system . _ , width=245 ]    the cdf experiment  @xcite has searched for the exclusive process @xmath94 and the invariant mass of the dimuon - photon system for candidate events is shown in figure  [ fig : higgs ] . under the assumption that all found events originate from exclusive @xmath91 production , the measured cross section was estimated to be @xmath95 , which is far from the expected number . i am very thankful to my colleagues from the hera and fnal experiments for their help in collecting the material for this talk . i would also like to acknowledge the hospitality of the max planck institute in munich , where this writeup was prepared , and the humboldt foundation for making my stay in munich possible . the research was partly supported by the israel science foundation . 99 v.  n.  gribov , jetp lett .   * 41 * , 667 ( 1961 ) . g.  ingelman and p.  e.  schlein , phys . * b152 * , 256 ( 1985 ) . f.  e.  low , phys . * d12 * , 163 ( 1975 ) . s.  nussinov , phys .  rev . lett .   * 34 * , 1286 ( 1975 ) . a.  donnachie and p.  v.  landshoff , phys . * b296 * , 227 ( 1992 ) [ hep - ph/9209205 ] . a.  donnachie and p.  v.  landshoff , nucl . * b231 * , 189 ( 1984 ) . k.  kang et al . , hep - ph/9812429 . j.  r.  cudell et al . d * 65 * , 074024 ( 2002 ) . j.  c.  collins , phys . rev .   * d57 * , 3051 ( 1998 ) [ hep - ph/9709499 ] ; 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recent experimental results on inclusive diffractive scattering and on exclusive vector meson production are reviewed . 
 the dynamical picture of hard diffraction emerging in perturbative qcd is highlighted . 
 = 14.5pt