This self-contained introduction to molecular astrophysics is suitable as a text for advanced postgraduate courses on interstellar matter. It is an excellent summary of present knowledge and outstanding questions and will be valued by research astrophysicists, physical chemists, atomic and molecular physicists and atmospheric scientists who wish to become familiar with this field. Descriptions are given of the distributions and types of molecules observed in galactic and extragalactic sources, including those in the vicinity of active galactic nuclei. The chemistry of diffuse and dense clouds is also discussed, and chemical reactions in shocks and dynamically evolving clouds are considered.