Granulated materials, like sand and sugar and salt, are composed of many pi
eces that can move independently. The study of collisions and flow in these
materials requires new theoretical ideas beyond those in the standard stat
istical mechanics or hydrodynamics or traditional solid mechanics. Granular
materials differ from standard molecular materials in that frictional forc
es among grains can dissipate energy and drive the system toward frozen or
glassy configurations. In experimental studies of these materials, one sees
complex flow patterns similar to those of ordinary liquids, but also freez
ing, plasticity, and hysteresis. To explain these results, theorists have l
ooked at models based upon inelastic collisions among particles. With the a
id of computer simulations of these models they have tried to build a "stat
istical-dynamics" of inelastic collisions. One effect seen, called inelasti
c collapse, is a freezing of some of the degrees of freedom induced by an i
nfinity of inelastic collisions. More often some degrees of freedom are par
tially frozen, so that there can be a rather cold clump of material in corr
elated motion. Conversely, thin layers of material may be mobile, while all
the material around them is frozen. In these and other ways, granular moti
on looks different from movement in other kinds of materials. Simulations i
n simple geometries may also be used to ask questions like When does the us
ual Boltzmann-Gibbs-Maxwell statistical mechanics arise?, What are the natu
re of the probability distributions for forces between the grains?, and Mig
ht the system possibly be described by uniform partial differential equatio
ns? One might even say that the study of granular materials gives one a cha
nce to reinvent statistical mechanics in a new context. [S0034-6861(99)0070
1-1].