A preponderance of theories have been adopted to identify the determinants of behavior. Despite claims of generalizability, research applying these theories has identified gaps or boundary conditions that delimit their application. Theory integration provides one means to address these gaps. This chapter outlines the contribution of integrated theories to advancing knowledge of behavior change. Four approaches to theory integration are identified and summarized: additional constructs, core constructs, expert consensus, and utility-based approaches. Theory integration is often motivated by the need to reduce redundancy in constructs across existing theories and improve their predictive power to arrive at optimally comprehensive, parsimonious, explanations of behavior. Examples of integrated theories are provided and how they have contributed to theory development outlined. Behavior change interventions based on integrated theory comprise multiple techniques that target change in the constructs that set the integrated theory apart from its constituent theories. While some interventions based on integrated theories have demonstrated efficacy in changing behavior, future research on integrated theories needs to adopt factorial designs that independently target change in integrated theory constructs. Such research will demonstrate the independent and interactive effects of techniques based on the integrated theory on behavior and on measures of the targeted theory construct.