Regret is fundamentally motivational, where there is a desire to be able to undo what has or has not been done; anticipating regret, therefore, seems a useful way of avoiding this negative experience and has been found to be a powerful mechanism for changing behaviour in areas such as economics and litigation (e.g. Larrick & Boles, 1995). Recent research has also been conducted into incorporating this explicitly affective component into the utilitarian-bound Theory of Planned Behaviour (Ajzen, 1985, 1988, 1991). A meta-analysis conducted in this thesis on all previous relevant research established that anticipated affective reactions, including regret, did make a significant contribution to the model in terms of increasing the variance in intentions. The remit of this PhD was to specifically consider anticipated regret’s predictive value to a broader variety of behaviours in a completely novel way. Study 1 differentiated between two particular behaviour types, Immediate Hedonic and Distal Benefit Behaviours, which mapped on to action and inaction regret respectively. A selection of these behaviours was used in Studies 2, 3 and 4, with Study 4 using an intervention designed to explicitly increase regret salience. Studies 5 and 6 set out to replicate the results from previous research into exercise behaviour by Abraham and Sheeran (2003; 2004), whilst Study 7 used an objective measure of exercise behaviour. The studies were either traditional pen-and-paper or web studies. The results provide overwhelming evidence that anticipated regret (unambiguously defined) adds significantly and independently to the predictive validity of the TPB in terms of intentions over and above the traditional TPB variables, even when controlling for past behaviour, experiential and instrumental attitudes. Replication of Abraham and Sheeran’s (2003; 2004) results (i.e. moderation by anticipated regret on the intention-behaviour relationship, mediated by intention stability) failed in pen-and-paper Study 5, although success was evident in web Study 6. Finally, although Study 7 explicitly established the disparate value of anticipated regret to the predictive validity of intention, there was a failure to detect any moderation or mediation effects: rather the most important predictor of exercise behaviour by Sports Centre members was past behaviour. Issues regarding multi-item versus single-item scales, web versus pen-and-paper designs, student versus ‘other’ populations, and observational designs are discussed with recommendations for further research