This book provides a comprehensive, theory-based analysis of current issues in population economics. It addresses the most important problems caused by demographic changes using the popular overlapping generations growth model by Samuelson and Diamond. Taking into account families' fertility decisions, it examines not only the demographic changes due to longer life expectancy but also the effects of social security policy on demography and labor supply/individual retirement behaviors. Conducting all analyses in a dynamic general equilibrium setting, the book offers a valuable theoretical reference guide in the field of population economics.