Understanding the brain mechanisms which underlie behavior is one of the most challenging tasks of modern biology. The study of these mechanisms can be approached using a variety of biological systems as models, depending essentially on the type of question being asked and the technical approach which is considered. In vertebrates, the study of brain neurochemistry in relation to behavior e,xpression has made tremendous progress during the last two decades. In particular, much at tention has been devoted to the effect of steroid hormones on brain structure and activity in connection with social and mainly reproductive behavior. This book exemplifies some of the major trends in the field. I did not attempt to cover exhaustively all the work that has been done in this area but rather to present a series of selected reviews on the molec ular and cellular brain mechanisms most directly related to social behavior. This selection obviously reflects my own interests but also, I believe, highlights those areas of research in which important progress has been made in recent years. A number of brain biochemical or cellular mechanisms which could be related to behavior, but have not been formally demonstrated to be so, are consequently ignored. The focus here is on social behavior and thus steroid-dependent processes are covered in priority. This choice of course leads us to ignore major trends in brain and behavior research but this is, I hope, clearly reflected in the title of the volume.