Increasing evidence suggests that public health and health-promotion interventions that are based on social and behavioral science theories are more effective than those lacking a theoretical base. This article provides an overview of the state of the science of theory use for designing and conducting health-promotion interventions. Influential contemporary perspectives stress the multiple determinants and multiple levels of determinants of health and health behavior. We describe key types of theory and selected often-used theories and their key concepts, including the health belief model, the transtheoretical model, social cognitive theory, and the ecological model. This summary is followed by a review of the evidence about patterns and effects of theory use in health behavior intervention research. Examples of applied theories in three large public health programs illustrate the feasibility, utility, and challenges of using theory-based interventions. This review concludes by identifying cross-cutting themes and important future directions for bridging the divides between theory, practice, and research.