Decisions about when to start or to stop antiepileptic drug (AED) treatment must be informed by reliable information of the natural history of epilepsy, the effect of treatment, and the social context of the individual. Ultimately the patient will be the decision maker, the clinician or health-care professional, the provider and interpreter of information. While observational studies will provide information on natural history, the most reliable information on the effect of intervention will come from randomised controlled trials in relevant populations of patients. However, these need to be large enough to allow interpretation not just of the average effect across the recruited patients, but also some estimate of the effects for an individual based on the prognostic effects that most effect outcomes. When trials are of sufficient size they can allow the development of predictive models that assist decision making. The Medical Research Council studies of AED withdrawal and early epilepsy and single seizures provide examples of such trials.