The present volume has been designed to be a self-contained introduction to the field of molecular astrophysics. It can serve as the text for a one semester postgraduate course concerning that subject exclusively or as a supplementary text in a postgraduate course on the interstellar medium. It can also be used by research astronomers, atomic and molecular physicists, chemists, and atmospheric scientists who have interests in weakly ionized plasmas and the physical and chemical processes which occur in them and who wish to become familiar with recent work in molecular astrophysics.

Many of the articles concern theoretical studies and modelling. Part I, consisting of two chapters written by observers, provides a general description of the astronomical context within which much of the remainder of the volume should be considered. The contribution by Per Friberg and Åke Hjalmarson reviews briefly our understanding of the global physical properties of the Galactic interstellar medium, describes the wide range of conditions within the dark molecular clouds (those having visual extinctions greater than about 1) and the distribution in the Galaxy of the dark clouds, and compares the chemical contents of different regions in various dark clouds. Leo Blitz has written about observations of molecules in other galaxies including some with active nuclei.

The study of chemistry in diffuse molecular clouds (those having a visual extinction of about 1 or less) constitutes the subject of Part II and is perhaps the most fundamental area in molecular astrophysics.