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knowledge of the mass distribution within clusters , ( also in relation to the distributions of the different cluster components ) , gives important clues about the formation process of the clusters and of the galaxies in them , as well as on the nature of dark matter . there have been many studies of the mass distribution in galaxy systems over the last decade , stimulated by the discovery of the universal nfw mass density profile of dark matter haloes by navarro et al . ( 1996 , 1997 ) . a cluster mass distribution can be derived in three ways : 1 ) through the gravitational lensing of distant objects , 2 ) using the spatial distribution and temperature profile of the x - ray emitting , intra - cluster ( ic hereafter ) gas , and 3 ) through the kinematics and spatial distribution of tracer particles moving in the cluster potential . lensing mostly works for clusters at intermediate and large redshifts , and only few nearby cluster lenses are known ( see e.g. cypriano et al . x - ray observations only sample the inner ( see , e.g. , pratt & arnaud 2002 ) or , at best , the virialized ( neumann 2005 ) cluster regions . the third method is particularly suited for studying the mass profiles of relatively nearby galaxy clusters , out to large radii , well beyond the virialized region ( see , e.g. , reisenegger et al . 2000 ) . additionally , using galaxy distributions it is also possible to constrain the velocity anisotropies , and hence the orbits , of cluster galaxies ( see , e.g. , biviano & katgert 2004 , bk04 hereafter ) . in this paper , i review recent results on cluster mass profiles as obtained from the analysis of the spatial and velocity distributions of cluster member galaxies ( see biviano 2002 for another recent review on this topic ) . in order to determine a cluster mass profile , @xmath0 , using the projected phase - space distribution of its member galaxies we can use the jeans analysis ( see , e.g. , binney & tremaine 1987 , bt87 hereafter ) , or the caustic method recently introduced by diaferio & geller ( 1997 ; see also diaferio 1999 ) .    in the jeans analysis the observable projected phase - space distribution of galaxies is related to the cluster @xmath0 , using the abel and jeans equations ( eqs . 4 - 55 , 4 - 57 , and 4 - 58 in bt97 , for the simplified case of a stable , non - rotating , spherically symmetric system ) , through knowledge of the velocity anisotropy profile , @xmath1 , which characterizes the orbits of cluster galaxies .    in the caustic method , one infers @xmath0 of a given cluster from the amplitude of the caustics in the plane of line - of - sight velocities vs. projected clustercentric distances . formally , the caustic amplitude is related to the gravitational potential through a function @xmath2 of the potential itself , and of @xmath1 , ( see eqs . 9 and 10 in diaferio 1999 ) . numerical simulations indicate @xmath3 , but only at radii larger than the cluster virial radius , @xmath4 ( see fig .  2 in diaferio 1999 ) . hence , for @xmath5 the caustic mass estimate is not very accurate . on the other hand , since the method does not rely on the assumption of dynamical equilibrium , it is a very powerful tool to constrain @xmath0 at large radii . note that both with the jeans and the caustic method one samples the _ total , _ not the _ dark _ mass of a cluster . this must be taken into consideration when observational results are compared with results from numerical simulations of collsionless dark matter haloes . using x - ray data it is possible to subtract the baryonic component of the total mass and infer the dark mass distribution ( see okas & mamon 2003 , lm03 hereafter ) , but this has not been done very often . a fundamental problem of the jeans analysis ( and , to a lesser extent , also of the caustic method ) is the mass  orbit degeneracy , i.e. the solution obtained for @xmath6 is degenerate with respect to the solution obtained for @xmath1 . in order to break the degeneracy , @xmath1 must be constrained independently from @xmath0 . this can be achieved via the analysis of the _ shape _ of the galaxy velocity distribution , that contains the required information about the orbital anisotropy of cluster galaxies ( merritt 1987 ; van der marel et al . 2000 , vdm00 hereafter ; lm03 ) . alternatively , if several tracers of the gravitational potential are available , the jeans equation can be solved for @xmath0 independently for each of the tracers , thus restricting the range of possible solutions ( bk04 ) . another relevant problem in the jeans analysis occurs if the cluster is not in steady state and dynamical equilibrium . since clusters grow by accretion of field galaxies ( e.g. moss & dickens 1977 ) , they are not steady - state systems . formally , inclusion of the time derivative in the jeans equation ( bt87 , eq . 4 - 29c ) is then needed . on the other hand , the fractional mass infall rate is estimated to be quite negligible for nearby clusters ( ellingson et al . moreover , most of the mass is accreted in big , discrete clumps ( zabludoff & franx 1993 ) . hence , those clusters undergoing substantial mass accretion can be identified through the presence of substructures in their galaxies distribution , and excluded from the analysis ( vdm00 ; biviano & girardi 2003 ) . the problem gets tougher when one is dealing with small galaxy systems ( galaxy groups ) most of which are probably still in a pre - virialized collapse phase ( giuricin et al . 1988 ) . interlopers are another potential problem in cluster mass estimates . cluster velocity dispersion estimates have been shown to be robust vs. modifications of the method of interlopers removal ( girardi et al . 1993 ) , but estimates of the kurtosis of the velocity distribution ( needed to constrain @xmath7 , see lm03 ) are not . hence , it is advisable to use other , more robust , estimators of the shape of a cluster velocity distribution ( e.g. the gauss - hermite moments , vdm00 ) . as the number of available redshifts for cluster galaxies increases , the interloper selection procedure becomes more robust . since in practice spectroscopic samples of more than 100 member galaxies per cluster are rare , quite often a composite cluster is built by stacking together the data for several clusters ( see , e.g. , carlberg et al . 1997 ; vdm00 ; katgert et al . the procedure is justified if clusters form a homologous family , as suggested by the existence of a fundamental plane of cluster properties ( schaeffer et al . 1993 ; adami et al . 1998 ) .    in the jeans analysis it is generally assumed that clusters are spherical and do not rotate . the composite cluster is spherically symmetric by construction , and deviation from spherical symmetry is unlikely to be a major problem for individual clusters either ( see vdm00 ; sanchis et al . 2004 ) . while evidence for cluster rotation has been claimed for a couple of clusters ( see , e.g. , biviano et al . 1996 ; dupke & bregman 2001 ) , the energy content in the rotational component is marginal . finally , dynamical friction and galaxy mergers could in principle invalidate the use of the _ collisionless _ jeans equation . however , the cluster velocity dispersion is too high for galaxy mergers to take place , and the dynamical friction timescale is too long for most cluster galaxies , except for very massive galaxies ( e.g. biviano et al . these can be removed from the sample ( katgert et al . 2004 ) prior to application of the jeans analysis . galay mergers in low - velocity dispersion systems , like groups , can however be a critical issue . since groups contain only a small number of galaxies each , their mass profile can only be inferred by stacking several of them together . results so far are contradictory . mahdavi et al . ( 1999 ) used 588 galaxies in 20 groups to conclude that their mass density profile is consistent with a hernquist ( 1990 ) model , and that the mass - to - galaxy number - density profiles ratio is constant . mahdavi & geller ( 2004 ) used the rasscals sample ( 893 galaxies in 41 nearby groups ) to determine a single power - law mass density profile , @xmath8 over a wide radial range ( 0@xmath9 ) . finally , carlberg et al . ( 2001 ) analysed @xmath10 galaxies in @xmath11 groups from the cnoc2 survey , at redshifts @xmath120.55 . they found a cored mass density profile near the groups centres , decreasing outside with a shallow slope , hence implying a steeply increasing mass - to - light ratio with radius .    despite these different results , all these studies conclude that galaxies in groups have , on average , nearly isotropic orbits , except late - type galaxies that are on mildly radial orbits ( mahdavi et al . 1999 ) . the coma cluster , with its nearly 900 spectroscopically confirmed cluster members ( adami et al . 2005 ) , is probably the best studied cluster in the universe . leaving apart historical studies ( see biviano 1998 and references therein ) , the first modern analysis of the coma cluster mass profile was done by merritt & saha ( 1993 ) . they found that the density profile is cuspy near the centre , @xmath13 , and decreases as @xmath14 , at large radii , if the orbits are isotropic . more recently , lm03 found that the nfw profile provides a good fit to the derived mass distribution , but other models are acceptable , and even a cored profile near the centre can not be excluded . at variance with merritt & saha , and unlike most other studies , lm03 analysed the _ dark , _ not the total , mass profile of the cluster . using the kurtosis profile of the galaxies velocity distribution , they obtained @xmath15 for early - type galaxies , i.e nearly isotropic orbits . the above results are in agreement with those obtained through the caustic method ( geller et al . 1999 ; rines et al . 2001 ) . these studies also constrained the behaviour of the mass density profile at very large radii , @xmath16 with @xmath174 . the isothermal sphere model ( @xmath18 ) is rejected , while both a nfw and a hernquist ( 1990 ) model are acceptable . the mass - to - light ratio in the @xmath19-band is found to be nearly constant out to @xmath20 .    in a series of papers , rines et al . ( 2000 , 2003 , 2004 ) have applied the caustic technique to another 8 nearby clusters of the cairns survey . the results are not very different from those found for the coma cluster . typically , @xmath21 near the centre , and the best - fit asymptotic slope of the mass density profile at large @xmath22 is either @xmath23 ( nfw ) or @xmath24 ( hernquist 1990 ) . when the nfw profile is adopted , the best - fit values of the concentration parameter @xmath25 vary between 5 and 17 . using the mass profiles obtained from the caustic technique in the jeans analysis , @xmath26 is obtained . the mass - to - light ratio in the @xmath19 band is approximately constant out to @xmath4 , then decreases by a factor @xmath27 out to the turnaround radius .    following up a previous study by carlberg et al . ( 1997 ) , vdm00 stacked together 16 clusters at @xmath280.55 from the cnoc survey , to build a composite cluster with @xmath29 galaxies . through the analysis of the shape of the galaxies velocity distribution , vdm00 were able to constrain @xmath30 . from the jeans analysis they then determined a mass profile which is fully consistent with a @xmath31 nfw model . the ratio between the mass density profile and the galaxies number density profile was found to be nearly constant . biviano & girardi ( 2003 ) analysed a composite cluster of 1345 member galaxies , obtained from the stacking of 43 clusters from the 2dfgrs . they performed a joint jeans and caustic analysis , using the former method to determine the mass profile in the virialized region ( @xmath32 ) , and the latter to extend @xmath0 to @xmath33 . i.e. they used each of the two methods in the cluster region where it is expected to perform best . they found that a @xmath34 nfw profile fits the data over the whole radial range explored . cored profiles are also acceptable but only for core radii @xmath35 . the ratio of the mass density to the galaxy number density profiles was found to be constant out to @xmath4 , and to decrease beyond that , mostly because of the contribution of late - type galaxies in the external regions .    , of 59 clusters from the enacs ( points with 1-@xmath36 error bars ) . the solid , dot - dashed , and dashed lines are the best - fit nfw , burkert ( 1995 ) , and softened isothermal sphere models , respectively , width=453 ]    katgert et al . ( 2004 ) analysed a composite of 1129 galaxies from 59 nearby clusters from the enacs . @xmath0 was determined in a fully non - parametric way . only early - type galaxies were used in the analysis . their velocity distribution was showed to imply nearly isotropic orbits , hence @xmath37 was adopted in the jeans analysis . the resulting mass density profile has a slope of @xmath38 at @xmath39 , fully consistent with an asymptotic slope of @xmath23 at larger radii . both a @xmath40 nfw profile , and a burkert ( 1995 ) profile with a rather small core ( @xmath41 ) are acceptable ( see figure  1 ) . the ratio of the mass - to - luminosity density profiles ( in the @xmath42 band ) is nearly constant out to @xmath43 , when both the cd - like galaxies ( that give an excess luminosity near the centre ) , and the late - type galaxies ( that give an excess luminosity in the outer regions ) are excluded . the @xmath0 solution obtained for the early - type galaxies was later confirmed by using a different tracer of the gravitational potential , early - type _ spirals _ ( sa  sb ; bk04 ) . inverting the jeans equation ( via the method of solanes & salvador - sol 1990 , developed from binney & mamon 1982 ) , bk04 found that also early - type spirals have nearly isotropic orbits in the cluster potential . on the other hand the late spirals ( sbc  sd ) plus irregulars have @xmath26 only near the centre , and increasingly radial orbits outside . galaxies in substructures were found to have tangential orbits at all radii . work is in progress ( biviano & salucci in preparation ) to determine the relative distributions of the different mass components , namely the baryonic matter ( in galaxies and in the intra - cluster gas ) , the dark matter in subhaloes , and the diffuse dark matter . preliminary results indicate that once the baryons and the dark matter subhaloes are subtracted from the total mass profile , the remaining diffuse dark matter profile is still consistent with both a burkert ( 1995 ) and a nfw model , but the concentration is a factor of two higher than for the _ total _ mass profile . the galaxy baryonic mass profile is found to be similar to the total mass profile , but the total baryonic mass is more widely distributed , because of the dominant contribution by the ic gas . superclusters are not virialized systems . it is then impossible to determine their mass profile other than by the use of the caustic method . reisenegger et al . ( 2000 ) and rines et al . ( 2002 ) applied this method to the shapley and the a2197/a2199 supercluster , respectively . the superclusters mass profiles are well fit by both a nfw and a hernquist ( 1990 ) model . useful constraints can be put on the mass distribution within clusters from the analysis of the projected phase - space distribution of their member galaxies . the constraints are poor near the centre , @xmath44 with @xmath45 , because of an intrinsic finite resolution ( the size of the cd galaxy ) . cored profiles are allowed , but only for core sizes below this resolution size , @xmath46 . stronger constraints are obtained at large radii , and they are stronger than those obtained with any other methods , @xmath47 with @xmath48 . in summary , the isothermal sphere is ruled out , while the nfw , burkert ( 1995 ) and hernquist ( 1990 ) models are acceptable . when comparing a cluster mass density profile with the galaxy number ( or luminosity ) density profiles , it must be realized that the result of this comparison depends upon which cluster galaxy population ( or , almost equivalently , which photometric band ) is considered . more relevant is the comparison of the total mass distribution with the baryonic mass distribution . the galaxy baryonic mass profile is similar to the total mass profile , but the total baryonic mass distribution ( including the ic gas ) is more extended .    as fas as the orbits are concerned , ellipticals , lenticulars , and early - type spirals ( sa  sb ) are all found to be on nearly isotropic orbits . late - type spirals ( sbc - sd ) and irregulars have increasingly radial velocity anisotropy with increasing radius , and galaxies in substructures seem to be characterized by tangential orbits . the implications of these results for the evolution of galaxies in clusters is discussed in bk04 . very little is known about the evolution of cluster dynamics with redshift . the average mass profile and the orbital characteristics of galaxies of nearby clusters ( enacs , cairns , 2dfgrs ) are similar to those of medium - distant clusters ( cnoc ) . hence , little or no evolution over @xmath490.5 is implied ( see also girardi & mezzetti 2001 ) .    in the near future it will be possible to use the sdss products ( see e.g. goto 2005 ) to reduce the current uncertainties on the dynamics of nearby clusters . it is also urgent to improve our understanding of the dynamics of small galaxy systems ( groups ) . a promising data - set in this respect is the gems groups sample of osmond & ponman ( 2004 ) , for which also x - ray temperatures are available . these can provide robust scaling parameters for the build - up of a composite group sample . determining the average mass profile ( and galaxy orbits ) of distant clusters ( @xmath50 ) is a more demanding task . distant clusters are likely to be in a more turbulent phase of their evolution than nearby clusters , making the dynamical analysis even more difficult . as a consequence , a deeper sampling of the projected phase - space distribution of member galaxies is needed , with a strong investment in terms of observing time ( e.g. demarco et al . 2005 ) . adami , c. , biviano , a. , durret , f. , & mazure , a. 2005 , astro - ph/0507542 adami , c. , mazure , a. , biviano , a. , katgert , p. , & rhee , g. 1998 , a&a , 331 , 493 binney , j. , & mamon , g. 1982 , mnras , 200 , 361 binney , j. , & tremaine , s. 1987 , galactic dynamics , princeton univ . press ( bt87 ) biviano , a. 1998 , in untangling coma berenices : a new vision of an old cluster , eds . mazure , a. , casoli , f. , durret , f. , & gerbal , d. , world scientific , also astro - ph/9711251 , and http://nedwww.ipac.caltech.edu/level5/biviano/frames.html biviano , a. 2002 , in tracing cosmic evolution with galaxy clusters , eds . borgani , s. , mezzetti , m. , valdarnini , r. , asp conf . series 268 ; 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the mass distribution of galaxy clusters can be determined from the study of the projected phase - space distribution of cluster galaxies . 
 the main advantage of this method as compared to others , is that it allows determination of cluster mass profiles out to very large radii . here 
 i review recent analyses and results on this topic . 
 in particular , i briefly describe the jeans and caustic methods , and the problems one has to face in applying these methods to galaxy systems . 
 then , i summarize the most recent and important results on the mass distributions of galaxy groups , clusters , and superclusters . 
 additional covered topics are the relative distributions of the dark and baryonic components , and the orbits of galaxies in clusters .