Theory has an integral role in the design of interventions and studies aimed at evaluating the effects of interventions consisting of single or multiple components. It also guides sample selection, operationalization of the intervention, and the specification of outcomes. Results of theory-based intervention evaluation indicate who benefits from the intervention, delivered in what context, and how the intervention and/or its components produce(s) changes in the outcomes. This knowledge is useful for translating research findings to practice and in making relevant decisions regarding the application of the intervention and/or its components in the practice setting. This chapter reviews the role of theory in intervention studies and briefly discusses strategies for developing interventions. It clarifies and illustrates the contribution of theory to the design of interventions and pertinent evaluation studies with examples. Finally, the chapter identifies and illustrates sources for generating theory with examples from the literature.