Objective: Theory use may enhance effectiveness of behavioral interventions, yet critics question whether theory-based interventions have been sufficiently scrutinized. This study applied a framework to evaluate theory use in physical activity interventions for breast cancer survivors. The aims were to (1) evaluate theory application intensity and (2) assess the association between extensiveness of theory use and intervention effectiveness. Methods: Studies were previously identified through a systematic search, including only randomized controlled trials published from 2005 to 2013, that addressed physical activity behavior change and studied survivors who were <5 years posttreatment. Eight theory items from Michie and Prestwich's coding framework were selected to calculate theory intensity scores. Studies were classified into three subgroups based on extensiveness of theory use (Level 1 = sparse; Level 2 = moderate; and Level 3 = extensive). Results: Fourteen randomized controlled trials met search criteria. Most trials used the transtheoretical model (n = 5) or social cognitive theory (n = 3). For extensiveness of theory use, 5 studies were classified as Level 1, 4 as Level 2, and 5 as Level 3. Studies in the extensive group (Level 3) had the largest overall effect size (g = 0.76). Effects were more modest in Level 1 and 2 groups with overall effect sizes of g = 0.28 and g = 0.36, respectively. Conclusions: Theory use is often viewed as essential to behavior change, but theory application varies widely. In this study, there was some evidence to suggest that extensiveness of theory use enhanced intervention effectiveness. However, there is more to learn about how theory can improve interventions for breast cancer survivors.