Changing behavior is a central challenge in domains ranging from health and finance to voting and education. Drawing on decades of research from psychology and related fields, this review synthesizes high-quality evidence—particularly randomized controlled trials with objective behavioral outcomes—to identify effective strategies for changing behavior. We propose a unifying three-phase framework that conceptualizes behavior change as a process involving: (a) building motivation, (b) following through, and (c) forming durable habits. For each phase, we identify key psychological challenges and review interventions designed to overcome them. By organizing a large and fragmented literature within a coherent framework, this review aims to clarify when and why behavior change interventions succeed and to inform more effective, behaviorally informed policies and practices.