There is limited evidence depicting ways that behavioral theory and techniques have been incorporated into cancer rehabilitation interventions. Examining their use within cancer rehabilitation interventions may provide insight into the active ingredients that can maximize patient engagement and intervention effectiveness. This secondary analysis aimed to describe the use of behavior change theory and behavior change techniques (BCTs) in two previously conducted systematic reviews of cancer rehabilitation interventions. Secondary analysis of randomized controlled trials (RCTs) drawn from two systematic reviews examining the effect of cancer rehabilitation interventions on function and disability. In-person and remotely delivered rehabilitation interventions. Adult cancer survivors. Data extraction included: behavior change theory use, functional outcome data, and BCTs using the Behavior Change Technique Taxonomy (BCTTv1). Based on their effects on function, interventions were categorized as "very", "quite" or "non-promising". To assess the relative effectiveness of coded BCTs, a BCT promise ratio was calculated (the ratio of promising to non-promising interventions that included the BCT). Of 180 eligible RCTs, 25 (14%) reported using a behavior change theory. Fifty-four (58%) of the 93 BCTs were used in least one intervention (range 0-29). Interventions reporting theory use utilized more BCTs (median=7) compared to those with no theory (median=3.5; U=2827.00, P=0.001). The number of BCTs did not differ between the very, quite, and non-promising intervention groups (H(2)=0.24, P=0.85). 20 BCTs were considered promising (promise ratio >2) with goal setting, graded tasks, and social support (unspecified) having the highest promise ratios. While there was a wide range of BCTs utilized, they were rarely based on theoretically-proposed pathways and the number of BCTs reported was not related to intervention effectiveness. Clinicians should consider basing new interventions upon a relevant behavior change theory. Intentionally incorporating the BCTs of goal setting, graded tasks, and social support may improve intervention efficacy.