Astrophysical gases are often highly inhomogeneous, with two or more "thermal phases" coexisting in rough pressure balance with one another. Compared to the scales of typical inhomogeneities, the transitions between neighboring regions of different temperature (and density) can be quite sharp. Such systems are most often modeled as consisting of cold clouds, filaments or sheets embedded in a hotter intercloud medium, although there may be cases in which a model consisting of hot bubbles in a cold matrix is more appropriate. Usually the topology of the phases is highly uncertain, but the conditions which lead to their existence are more reliably established. The temperatures of the phases sometimes differ by orders of magnitude, and are frequently set within rather narrow ranges by the details of atomic and molecular processes or by the spectrum of ambient radiation. It should be stressed that thermal pressure balance may not be exact, e.g., where magnetic fields or cosmic rays supply a significant fraction of the pressure in one or more phases, where self-gravity or turbulent pressure are dynamically important, where ram pressure (associated with differential motion of the phases) provides part of the confinement, in the case of a cool cloud evaporating suprathermally in a hot background (Balbus and McKee 1982), or when there is simply too little time for a system to achieve dynamical equilibrium. Although the concepts of multiphase media are not generally used to describe regions which are wildly out of dynamical equilibrium with their surroundings (such as material behind a propagating shock front), localized pressure fluctuations may be an important means of transferring material between phases (Wang and Cowie 1988).
This review presents an overview of the theory of thermal phases, with particular attention to their role in the interstellar media of galaxies. In Section 2 I discuss the basic reasons for the existence of multiphase media, and show the connection between multiple phases and thermal instability. I also give examples of multiphase systems which are important in astrophysics. Since the phases are in physical contact, it is unrealistic to treat them as being isolated from one another. Section 3 deals with the principal interactions among phases, thermal conduction and ablation. Taking these interactions into account is particularly important if one wishes to understand the temporal evolution of multiphase media; Section 4 deals with the consequences of mass exchange and with evolutionary models. The "state of the art" is summarized in Section 5. Much of the original material presented in this review was developed in collaboration with C. F. McKee, and is described in greater detail in Begelman and McKee (1990).