The use of antiepileptic drugs has become increasingly effective through several factors: new techniques that allow better diagnosis of the seizure disorder and its underlying cause; the development of new medications and increased knowledge of old ones; and the widespread use of antiepileptic drug-level determinations. The choice of a drug depends heavily on an accurate diagnosis of seizure type, which may determine the response to the medication. Because of better diagnostic criteria and intensive monitoring procedures, the correct seizure disorder can be more easily diagnosed and, therefore, the proper medication selected. Minimal efficacious and toxic blood concentrations have now been identified for most antiepileptic drugs. Several, including primidone, carbamazepine, methsuximide, and mephenytoin, have pharmacologically active metabolites that affect both the toxicity and efficacy of the prescribed drug and can now be measured in the plasma. The most effective use of the antiepileptic drugs depends on a combination of reliable blood level measurements, clinical observation, and knowledge of their pharmacokinetics and biotransformation.