Interventions are increasingly described as theory-based; however, the basis for this is often not clear. Advancing behavioral science requires a good understanding of how interventions are informed by, and test, theory. This study aims to develop a reliable method for assessing the extent to which behavioral interventions are theory-based. The reliability, usability, and comprehensiveness of an initial coding scheme were improved in 13 iterative stages on the basis of its application to 29 papers, from a systematic review of interventions to promote physical activity and healthy eating. The final Theory Coding Scheme contained 19 items, each with satisfactory interrater reliabilities, coding whether a theory or model was mentioned, how theories were used in intervention design, how intervention evaluations tested theory, and the implications of the results for future theory development. The Theory Coding Scheme is an important methodological innovation, providing a research tool to reliably describe the theoretical base of interventions, inform evidence syntheses within literature reviews and meta-analyses, and stimulate the use of empirical data for theory development.(PsycINFO Database Record (c) 2016 APA, all rights reserved)