The subject of this book lies at the confluence of two major currents in contemporary science: phase transitions and far-from-equilibrium phenomena. It is a subject that continues to attract scientists, not only for its novelty and technical challenge, but because it promises to illuminate some fundamental questions about open many-body systems, be they in the physical, the biological, or the social realm. For example, how do systems composed of many simple, interacting units develop qualitatively new and complex kinds of organization? What constraints can statistical physics place on their evolution?

Nature, both living and inert, presents countless examples of nonequilibrium many-particle systems. Their simplest condition — a nonequilibrium steady state — involves a constant flux of matter, energy, or some other quantity (de Groot & Mazur 1984). In general, the state of a nonequilibrium system is not determined solely by external constraints, but depends upon its history as well. As the control parameters (temperature or potential gradients, or reactant feed rates, for instance) are varied, a steady state may become unstable and be replaced by another (or, perhaps, by a periodic or chaotic state). Nonequilibrium instabilities are attended by ordering phenomena analogous to those of equilibrium statistical mechanics; one may therefore speak of nonequilibrium phase transitions (Nicolis & Prigogine 1977, Haken 1978, 1983, Graham 1981, Cross & Hohenberg 1993).