This book deals with models that can capture the behavior of individuals and groups over time. Organizationally, it is divided into three parts. Part I discusses the basic, decrement-only, life table and its associated stable population. Part II examines multistate (or increment-decrement) models and provides the first comprehensive treatment of those extremely flexible and useful life table models. Part III looks at two-sex models, which simultaneously incorporate the marriage or fertility behavior of males and females. Those models are explored more fully and completely here than has been the case to date, and the importance of including the experience of both sexes is demonstrated analytically as weil as empirically. In sum, this book considers a broad range of population models with a view to showing that such models can be eminently calculable, clearly interpretable, and analytically valuable for the study of many kinds of social behavior. Four appendixes have been added to make the book more usable. Appendix A provides abrief introduction to calculus and matrix algebra so that readers can understand, though not necessarily derive, the equations presented. Appendix B provides an index of the principal symbols used. Appendix C gives the answers to the exercises found at the end of each chapter. Those exercises should be seen as an extension of the text, and are intended to inform as weil as to challenge.