The notion of optimum population has attracted the attention of economists ever since economics was made a science. Roots can be traced back to ancient Greece. The topic has recently found rising interest among population economists and demographers. The economic concept of optimum population seeks to define the population size, which maximizes a welfare criterion of the society. The purpose of this book is to outline this concept from a micro and macro perspective and to link it with issues of technical progress, social security, limited resources and migration. It treats fertility endogenously and studies its welfare and policy implications. The emphasis is on a rigorous theoretical treatment of the subject using the modern growth and welfare theory as well as the new classical micro model of the family.