Treating population growth as a dynamic process, this book considers quantitative studies of population in the framework of cybernetics, and describes some rules and characteristics of the population systems under control functions (population policy), thus establishing a scientific basis for population control and family planning. Particular features of this book are the combination of qualitative with quantitative studies, the combination of natural and social sciences and the combination of theory with practice. Differing from the demographic approach, this book uses the methods of systems science to study social systems, thereby putting forward a number of new concepts and theories, such as population system control model, population stability theory and population system optimal control. The book illustrates how the systems framework, systems methodology and systems theory represent a powerful scientific approach to some practical problems. The reader may also find here some new results reached by applying such methods to the study of population problems. The book in fact contains many new research findings that have not been published previously. The first author is Chairman of the State Science and Technology Commission of the People's Republic of China as well as Professor at Qinghua University. Prof. Song received the Albert Einstein Award from the International Association for Mathematical Modelling in August 1987. The second author is Deputy Director of the Beijing Institute of Information and Control. The book will be of great interest to population theorists and systems scientists, in particular as an account of work being done currently in China in this field.