  Nonequilibrium phase transitions between an active and an absorbing state are
found in models of populations, epidemics, autocatalysis, and chemical
reactions on a surface. While absorbing-state phase transitions fall
generically in the DP universality class, this does not preclude other
universality classes, associated with a symmetry or conservation law. An
interesting issue concerns the dynamic critical behavior of models with an
infinite number of absorbing configurations or a long memory. Sandpile models,
the principal example of self-organized criticality (SOC), also exhibit
absorbing- state phase transitions, with SOC corresponding to a particular mode
of forcing the system toward its critical point.
