Although the long-term processes of evolution are selection and mutation, the infrastructure of a population is a no less important force in determining the distributions of genetic characteristics observable within populations. In small populations, and in particular in human populations, complex patterns of genealogical relationship between individuals can be an important factor in the maintenance of genetic variability. The aim of this book is to develop the quantitative theory of the interrelationship between the genealogical and the genetic structures of a population. Aspects of other structural features, such as migration patterns, are also discussed, but are not central to the development. There are three major aspects; each comprises two chapters of the text. First, genealogical relationships are characterized in a way which can illuminate their genetic consequences. Second, the evolutionary aspects of genealogical structure are developed. Finally, the last two chapters present methods of characterizing the complete structure of a genealogy, and of computing relevant parameters of genealogical structure; these topics are of relevance to genetic epidemiology as well as to population genetics.