Phase III studies of antiepileptic drugs (AEDs) are specifically designed to satisfy strict regulatory criteria. As they are conducted in protocol-restricted patient populations over short treatment periods and employ fixed study designs and dosing schedules, they are not fully representative of ‘real-life’ clinical practice. Therefore, in order to provide an overall assessment of clinical performance, regulatory studies must be backed up by post-marketing clinical experience. Phase IV studies provide information on a drug's performance in a setting more closely representing real clinical practice, with broader patient populations and a more flexible approach to individual treatment. Prospective long-term studies allow the determination of efficacy and safety (and cost-effectiveness) over extended treatment periods; these studies and audit data provide a means of assessing idiosyncratic side effects, unusual interactions and the effects of an AED in rare patient groups. By complementing regulatory evidence with real-life clinical experience, a comprehensive assessment of the risks and benefits of an AED can be made.