The plan of this monograph is divided into four main parts. These parts develop in order of decreasing symmetry, from idealized infinite homogeneous systems to finite flattened irregular systems. Along this sequence, the ratio of model applications to fundamental physical ideas and techniques increases. Even so, I have tended to emphasize the basic physics over detailed applications. Specific astronomical models wax and wane as data and fashions change, but the principles on which they are built have much longer lifetimes. Thus the degree to which various topics are discussed does not always reflect their popularity in today's, or yesterday's, literature.

Nearly all the theory described in this book is based on classical Newtonian gravity. Relativistic generalizations of almost every aspect are possible, and there was a flurry of these generalizations in the 1960s and early 1970s. It was greatly stimulated by possible applications to quasars. Although quasars still are unsolved, no evidence has developed that relativistic star clusters are needed to explain them. That, plus the fact that there are enough fascinating things to say about observed non-relativistic systems, persuaded me to restrict this book to classical gravity.

The book is reasonably self-contained in that most of its results can be obtained directly from preceding ones, sometimes with two or three intermediate algebraic steps to be added by the reader. Usually these steps are straightforward and they are outlined in the text.