Cognitive-behavioral techniques have developed into the most thoroughly investigated psychological treatments for a wide range of illnesses since the advent of Beck's cognitive theory of emotional disorders and the treatment of those disorders with psychotherapy. Despite this, there is much disagreement on the relative contributions of cognitive and behavioural treatment modalities, as well as the mechanistic function of cognitive change in therapy. We conduct a critical assessment of the research in this area, concentrating on the mechanistic relevance of cognitive change in cognitive and behavioural treatments for anxiety and depressive disorders