A comprehensive overview of theoretical and observational understanding of the interstellar medium of galaxies.New space and ground-based observational opportunities have enabled rapid developments in our understanding of the molecular universe in recent years. This work provides a comprehensive overview of our current theoretical and observational understanding of the interstellar medium of galaxies. With emphasis on the microscopic physical and chemical processes in space, and their influence on the macroscopic structure of the interstellar medium of galaxies, the book includes the latest developments in this exciting area of molecular astrophysics. This is an invaluable reference source for advanced undergraduate and graduate students, and researchers.New space and ground-based observational opportunities have enabled rapid developments in our understanding of the molecular universe in recent years. This work provides a comprehensive overview of our current theoretical and observational understanding of the interstellar medium of galaxies. With emphasis on the microscopic physical and chemical processes in space, and their influence on the macroscopic structure of the interstellar medium of galaxies, the book includes the latest developments in this exciting area of molecular astrophysics. This is an invaluable reference source for advanced undergraduate and graduate students, and researchers.Emphasizing microscopic physical and chemical processes in space, and their influence on the macroscopic structure of the interstellar medium of galaxies, this book includes the latest developments in an exciting area of molecular astrophysics. New space and ground-based observational opportunities have resulted in significant additions to our knowledge of the molecular universe in recent years. This comprehensive overview will be an invaluable reference source for undergraduates, graduates and researchers.1. The galactic ecosystem; 2. Cooling processes; 3. Heating procel£.