The ambition to understand why and how psychotherapy works has guided theorists, researchers, and practitioners for decades. This has led to an accumulation of literature, both theoretical and empirical, exploring the mechanisms that facilitate change in the psychotherapeutic process. Over the past decades there has been considerable advancement in the areas of investigating the psychotherapy process (i.e., course of change, predictors of outcome, mediators and moderators of change). The primary aim of our paper is to draw attention to the importance of studying mechanisms of change and to delineate the most important theoretical and methodological milestones for evaluating the processes through which clinical change occurs. We first discuss prior work addressing mechanisms of change and argue how mechanisms are linked to other related concepts. We then outline several strategies in data analysis and briefly discuss their role in investigating mechanisms of change. Finally, we suggest key recommendations to be considered by researchers designing studies investigating mechanisms of change in psychological treatments, as well as recommendations for future research in this area.
KEYWORDS: psychotherapy, mechanism of change, mediation