Acceptance and Commitment Therapy for Major Depressive Disorder: Navigating Depression Treatment in Traditional and Digital Settings with Insights from Current Research.

Given the shortcomings of a mechanistic assumption of traditional cognitive behavioral therapy (CBT), a newer generation of CBT-grounded interventions focusing on process-orientated emotional and motivational aspects has emerged. These so-called third-wave CBTs emphasize function and context of inner experience over form and content, and have become evidence-based practice in the past four decades. Among these approaches, acceptance and commitment therapy (ACT) has both a large body of research for various (mental) health conditions, including major depressive disorder (MDD) in particular. ACT is a transdiagnostic approach that intends to increase psychological flexibility (PF) of clients as a universal mechanism of behavior change and a value-driven orientation in life. By focusing on present-moment awareness, acceptance, defusion, establishing a stable sense of self, clarifying personal valued life directions, and committing to behaviors consistent to these values, ACT targets the core processes of PF. Meta-analyses have indicated the efficacy and effectiveness of ACT in reducing depressive symptoms and increasing well-being, with mainly moderate effect sizes in clinical trials. ACT for MDD has been shown to be effective across different delivery modes (e.g., individual, group, digital). ACT can also be applied using self-help formats (e.g., mobile apps) and combined with features from compatible approaches like behavioral activation. There is also evidence for a high acceptability of ACT and adherence rates comparable to classic CBT. Moreover, process research has shown that ACT works specifically through the mediator of PF and by addressing its suggested core therapeutic processes. Given the essential role of offering a personalized therapeutic strategy in treatment outcomes and adherence, it is central to provide more effective options that match clients' needs and preferences. This chapter illustrates different applications of ACT for adults with MDD and the current evidence base to promote informed decisions on using ACT as additional or stand-alone therapeutic approach.