ACT vs CBT: an Exercise in Idiosyncratic Language James J. Collard 1 # Springer Nature Switzerland AG 2019 Abstract Since the development of acceptance and commitment therapy as a therapy model, there has been much debate about the similarities and differences between it and ‘second wave’ cognitive-behavioural practices (i.e. cognitive therapy, rational emotive behaviour therapy). This article builds on these discussions, discussing the underlying theoretical constructs relevant to the philosophy and practice of ACT and ‘second wave’ CBT models. Ultimately, this results in the conclusion that there are more similarities than differences between the two. Many of the espoused differences between them appear to be due to the implicit versus explicit attentional foci of the different models, and due to the development of idiosyncratic language by the different authors. As a result, it would appear that there is more to be gained by bringing these models together, to solidify the cognitive-behavioural framework and its application to therapy. Keywords Cognitive-behavioural therapy . ACT . REBT . CT Since the development of acceptance and commitment therapy (ACT), there have been a number of arguments put forth attempting to distinguish it from other cognitive behavioural therapies, particularly in relation to elements of underlying theory and philosophy (Hayes 2004, 2008; Hayes et al. 2013; Hayes et al. 1999). While some of these have been identified to be misconceptions (e.g. that traditional cognitive restructuring challenges rule governance by imposing further rules, not acceptance; Hayes 2008), there are a number of issues that would benefit from further exploration. This includes the similarities and differences between relational frame theory (RFT), the underlying philosophy of ACT and traditional cognitive-behavioural philosophy. It also relates to the exploration of the proposed theoretical mechanisms of change between second wave cognitive-behavioural therapy (CBT) and ACT. International Journal of Cognitive Therapy https://doi.org/10.1007/s41811-019-00043-9 * James J. Collard james.collard@cairnmillar.edu.au 1 School of Psychology, Counselling, & Psychotherapy, Cairnmillar Institute, Melbourne, Australia