The ability to measure the serum concentrations of antiepileptic drugs, and the widespread use of this procedure, has markedly improved the treatment given to patients with epilepsy during the past 3 decades. The monitoring of antiepileptic drug concentrations in serum is necessary for the optimal drug therapy of seizures, because the therapeutic and toxic effects of these drugs are better related to serum concentration than to administered dosage. Monitoring appeared to have a major impact on improving the effectiveness and safety of antiepileptic drug therapy.The age-related variability of pharmacokinetic parameters may also require the individualisation of therapy, with subsequent re-evaluation as the person grows older. Monitoring serum concentrations of antiepileptic drugs may help to optimise the dose. A drug concentration, however, can only be regarded as a guide around which to alter the dosage according to the patient's clinical condition.Serum drug concentration monitoring is particularly useful to ensure compliance and in helping to manage combinations of antiepileptic drugs that invariably interact. The addition or deletion of other antiepileptic drugs may change dosage requirements. Therefore, routine monitoring of antiepileptic drug serum concentrations would be extremely useful, especially in the paediatric population, and in patients who require associated antiepileptic medication.