This book presents an introduction to the problems and strategies for modeling age, period, and cohort (APC) effects for aggregate-level data. These strategies include constrained estimation, the use of age and/or period and/or cohort characteristics, estimable functions, variance decomposition, and a new technique called the s-constraint approach. Emphasizing both the geometry and algebra of several APC approaches, the book develops readers' understanding of the statistical issues of APC analysis and shows how common methods are related to each other.