Greater understanding of the mechanisms (mediators) by which behavioral-change interventions work is critical to developing theory and refining interventions. Although systematic reviews have been advocated as a method for exploring mediators, this is rarely done. One challenge is that intervention researchers typically test only two paths of the mediational model: the effect of the intervention on mediators and on outcomes. The authors addressed this challenge by drawing information not only from intervention studies but also from observational studies that provide data on associations between potential mediators and outcomes. They also reviewed qualitative studies of participants' perceptions of why and how interventions worked. Using data from intervention (n = 37) and quantitative observational studies (n = 55), the authors conducted a meta-analysis of the mediation effects of eight variables. Qualitative findings (n = 6) contributed to more in-depth explanations for findings. The methods used have potential to contribute to understanding of core mechanisms of behavioral-change interventions.