Epilepsy and its treatment can have deleterious cognitive and behavioural consequences. Affected individuals have a higher prevalence of neuropsychological dysfunction than the general population because of complex interactions among several multifaceted and overlapping influences--for example, underlying neuropathologies, ictal and interictal neuronal discharges, a plethora of antiepileptic drugs, and numerous psychosocial issues. Research into the clinical relevance of these factors has been dogged by a range of methodological pitfalls including lack of standardisation of neuropsychological tests, small numbers and multiple testing, and statistical failure to appreciate differential effects of interactive elements in individual patients. Although antiepileptic drugs can impair neuropsychological functioning, their positive effect on seizure control might improve cognition and behaviour. Each person should be assessed individually with respect to factors unique to his or her seizure disorder and its treatment.