OBJECTIVES: Comparison groups are essential for accurate testing and interpretation of yoga intervention trials. However, selecting proper comparison groups is difficult because yoga comprises a very heterogeneous set of practices and its mechanisms of effect have not been conclusively established. METHODS: We conducted a systematic review of the control and comparison groups used in published randomized controlled trials (RCTs) of yoga. RESULTS: We located 128 RCTs that met our inclusion criteria; of these, 65 included only a passive control and 63 included at least one active comparison group. Primary comparison groups were physical exercise (43%), relaxation/meditation (20%), and education (16%). Studies rarely provided a strong rationale for choice of comparison. Considering year of publication, the use of active controls in yoga research appears to be slowly increasing over time. CONCLUSIONS: Given that yoga has been established as a potentially powerful intervention, future research should use active control groups. Further, care is needed to select comparison conditions that help to isolate the specific mechanisms of yoga's effects.