Acceptance and Commitment Therapy (ACT) is a third-wave process-based cognitive-behavioural therapy, which has undergone significant empirical testing, through observational, experimental, and intervention research. ACT is an empirically supported intervention, with over 1000 randomised control trials, as well as associated meta-analyses, indicating efficacy and effectiveness across a range of problems for diverse populations. This chapter will provide readers with a high-level overview of the ACT outcomes literature. This includes examining where ACT has been shown to be effective (e.g., depression, chronic pain), while also highlighting where the evidence base is less established. To further contextualise this literature, we will outline the process-based empirical strategy that underpins ACT, guiding a longstanding focus on mechanisms of change grounded in basic science. The chapter then concludes with a discussion of the implications for clinicians of this literature review, as well as considering the future of ACT and psychological interventions more generally.