Cognitive behavioural therapy (CBT) has, in the space of 50 years, evolved into the dominant modality in psychological therapy. Mechanism/s of change remain unclear, however.

In this paper, we will describe key features of CBT that account for the pace of past and future developments, with a view to identifying candidates for mechanism of change. We also highlight the distinction between ‘common elements’ and ‘mechanisms of change’ in psychological treatment.

The history of how behaviour therapy and cognitive therapy developed are considered, culminating in the wide range of strategies which now fall under the heading of cognitive behavioural therapy (CBT). We consider how the empirical grounding of CBT has led to the massive proliferation of effective treatment strategies. We then consider the relationship between ‘common factors’ and ‘mechanisms of change’, and propose that a particular type of psychological flexibility is the mechanism of change not only in CBT but also effective psychological therapies in general.

Good psychological therapies should ultimately involve supporting people experiencing psychological difficulties to understand where and how they have become ‘stuck’ in terms of factors involved in maintaining distress and impairment. A shared understanding is then evaluated and tested with the intention of empowering and enabling them to respond more flexibly and thereby reclaim their life.