Published in "The Interstellar Medium in Galaxies", eds. H.A. Thronson, Jr. and J. Michael Shull, 1990 Kluwer Academic Publishers.

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Abstract. This review deals with the theory of multiphase media in astrophysical systems. I discuss the basic reasons for the existence of multiple thermal phases, and the fundamental connection between multiphase media and thermal instability. After describing important examples of multiphase media, I examine the interactions among phases, i.e., mass exchange driven by thermal conduction and hydrodynamic ablation. Mass exchange may compete with radiative heating and cooling for control of the thermal state of the hot phase, and may alter the thermal stability properties of the system.

Table of Contents

• INTRODUCTION
• THEORY OF MULTIPHASE MEDIA
• Why thermal phases?
• Connection with thermal instability
• Examples of thermal phases
• INTERACTIONS AMONG PHASES
• Evaporation and condensation
• Ablation
• Consequences of Mass Exchange
• Mass exchange vs. radiative heating/cooling
• Evolution of multiphase systems
• CONCLUSIONS
• REFERENCES