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