In 1988, Walter S. DeKeseredy announced Male Peer Support (MPS) Theory, which popularized the notion that certain all-male peer groups encourage, justify, and support the abuse of women. In 1993, DeKeseredy and Martin D. Schwartz modified and expanded MPS Theory. Today, after twenty-five years of research, numerous studies from a diverse range of fields and practitioners support the original claim, providing a powerful explanation for the mechanism that underlies much of North America’s violence against women. This book provides a history of the theory, traces its development and uses over a quarter century, and offers an update on Internet-generated abuse.