To appear in the Sept 2000 Encyclopaedia of Astronomy & Astrophysics, MacMillan and Institute of Physics Publishing

Table of Contents

GAS IN GALAXIES

ATOMIC AND MOLECULAR CLOUDS

THE INTERCLOUD MEDIUM

THE SOLAR NEIGHBORHOOD

THEORIES OF THE ISM

The Two-Phase Model

The Three-Phase Model

THE INFLUENCE OF SUPERNOVA

THE ROLE OF PULSARS

THE INFLUENCE OF MAGNETIC FIELDS

DEPENDENCE ON GALAXY TYPE

UNANSWERED QUESTIONS

BIBLIOGRAPHY