Acceptance and commitment therapy (ACT) is a transdiagnostic cognitive behavioural therapy that predominantly teaches clients acceptance and mindfulness skills, as well as values clarification and enactment skills. Australian treatment guideline providers have been cautious in recognising ACT as empirically supported. This article reviews evidence from randomised controlled trials published since Ost's review, and examines the extent to which the methodology of ACT research has improved since. Since 2008, ACT research has improved its use of adherence and competence monitoring. Good-quality studies could be considered to offer National Health and Medical Research Council Level II evidence for chronic pain, obsessive-compulsive disorder, and a subset of other anxiety disorders (panic disorder, social phobia, and generalised anxiety disorder). The majority of studies demonstrated that ACT significantly improved primary outcomes but used comparison conditions that did not rule out therapy-unspecific factors, including use of concurrent treatments, as explanations for the improvements. Recommendations for future ACT research are presented. (Contains 2 tables and 1 footnote.)