There is considerable interindividual variation in the relationship between control of seizures and the serum anticonvulsant concentration. The minimum effective serum concentration is dependent on the type and severity of the epilepsy, and varies from patient to patient. The therapeutic range should be used as a guide to adjust the dose in order to further improve seizure control or reduce toxicity; the latter is more likely with higher serum concentrations, but can also be present when concentrations are low. A request for the serum concentration of an anticonvulsant should be made only for good clinical reasons, and an interpretation of that concentration can only be made if all the relevant clinical details are available.Indications for the measurement of serum anticonvulsant concentrations include poor seizure control, toxicity, suspected gross noncompliance, status epilepticus and the elapse of 2 to 4 weeks after the initiation of therapy. Additional drug therapy, pregnancy or illness may alter drug disposition in a well controlled patient and therapeutic drug monitoring may, therefore, help to prevent seizures secondary to these changes. The measurement of anticonvulsants in saliva as opposed to serum may be of benefit in some patients.