Multiple thermal phases are known to exist in many astrophysical systems. The reasons for their existence are understood in general terms, but the detailed properties of specific multiphase systems are poorly known. We can look forward to the further development of multiphase models for the ISM in elliptical galaxies, cooling flows in clusters of galaxies, the intergalactic medium, and protogalactic environments. The structures of multiphase media are sensitive to the rate of mass exchange between phases, which tends to lower the temperature of the hot phase and render it thermally stable. Unfortunately, mass exchange through thermal conduction depends on the topology of conduction fronts and on the magnetic connectivity of the phases, neither of which is understood. However, mixing of the phases may be driven by hydrodynamic instabilities at a rate much faster than that due to thermal conduction. Rapidly improving hydrodynamic codes with a high dynamic range in spatial resolution (e.g., using an adaptive mesh) should clarify some of the physics of the ablation process within the next few years.
ACKNOWLEDGMENTS. Many of the ideas presented above were developed collaboratively with C. F. McKee. Portions of the text borrow heavily (and in some cases verbatim) from Begelman and McKee (1990), which is to be published in The Astrophysical Journal. Preparation of this article was supported in part by NSF grant AST88-16140, NASA Astrophysical Theory Center grant NAGW-766, and a grant from the Alfred P. Sloan Foundation.