This book is in effect a second edition of the book Galaxies: Structure and Evolution published by Wykeham Publications in 1978. When copies of the original edition were exhausted, the publishers were unwilling to reprint it. I am grateful to Dr Simon Mitton of the Cambridge University Press for agreeing to take the book over and for encouraging me to undertake the necessary task of revising the text.

The problem of the structure and evolution of galaxies is central to astronomy. On the one hand a galaxy is composed of stars, whose individual properties are known at least in broad outline. However, the process of star formation, which is crucial to the evolution of galaxies, is not at all well understood. On the other hand galaxies and clusters are the main constituents in the Universe and their properties provide important information about the origin and evolution of the Universe. In addition both the origin and present structure of galaxies are influenced by the possibility that the major form of matter in the Universe is not luminous stars but invisible weakly interacting particles.

In this book I discuss in general terms what is known both about the present structure of galaxies and about their past life history. Most of the detailed discussion refers to our own Galaxy. Although the subject is treated precisely where that is possible it will be apparent that, while the main ideas appear to be well-established, there are very considerable detailed uncertainties.