Statistical mechanics may be naturally divided into two branches, one
dealing with equilibrium systems, the other with nonequilibrium systems.
The equilibrium properties of macroscopic systems are defined in
principle by suitable averages in well-defined Gibbs's ensembles. This
provides a frame- work for both qualitative understanding and
quantitative approximations to equilibrium behaviour. Nonequilibrium
phenomena are much less understood at the present time. A notable
exception is offered by the case of dilute gases. Here a basic equation
was established by Ludwig Boltzmann in 1872. The Boltzmann equation
still forms the basis for the kinetic theory of gases and has proved
fruitful not only for a study of the classical gases Boltzmann had in
mind but also, properly generalized, for studying electron transport in
solids and plasmas, neutron transport in nuclear reactors, phonon
transport in superfluids, and radiative transfer in planetary and
stellar atmospheres. Research in both the new fields and the old one has
undergone a considerable advance in the last thirty years.