The cognitive bias of "Thought Action Fusion" (TAF) has received significant research attention in the past decade. The review addresses the assessment of TAF, its place in cognitive theories of obsessional difficulties, and the evidence demonstrating that TAF is relevant to disorders beyond Obsessive Compulsive Disorder (OCD). Data on the components of TAF, its extension to positive outcomes and its role in the aetiology, maintenance and treatment of OCD are reviewed. It is concluded that the moral form of TAF is less robust than the likelihood form and that scales may be best used as a starting point in identifying beliefs and conducting experimental investigations. It is also suggested that the scales be amended to include harm avoidance, which would also increase their clinical utility. (C) 2004 Elsevier Ltd. All rights reserved.