Book excerpt: This PhD. thesis reports eight studies into the determinants of ecstasy use behaviours. Although all prior quantitative studies into ecstasy use determinants have examined the generic behavioural category 'using ecstasy', these studies may not be able to inform the development of interventions aiming to prevent ecstasy use or to promote cessation. The studies reported in this thesis indicate that the key determinants of 'using ecstasy' are not also the key determinants of 'trying out ecstasy' and 'ceasing ecstasy use'. Furthermore, ecstasy use cessation usually seems to be prompted by life style changes (e.g. job/work, relationships, losing interest in the dance scene), rather than by fear of ecstasy's health effects. This means that health promotion efforts would do better to adopt a harm reduction approach, rather than to focus on abstinence. The current thesis reports the studies on which these conclusions are based, and the first study into the determinants of a harm reduction strategy.