Δ Theory establishes the foundation for a true scientific applied psychology, a theory of how human influence induces change in others. Δ Theory is unified and universal, applying to all cultures, historical periods, and goals for change. It integrates concepts and research from psychology, sociology, anthropology, evolution theory, philosophy, psychoneurology, cognitive science, and cultural-historical-activity theory. Yet Δ Theory is clear, economical, and elegant, with a full exposition of tactics for its practices. Rich examples are drawn from professional practices, but also from the creation and operations of criminals, healing ceremonies of indigenous peoples, and cross-species comparisons. This book ultimately seeks to describe how influence works, how it could be improved, and how it can be resisted.