This chapter reviews the pharmacokinetic principles that are important to drug interactions and relates these to the major mechanisms of drug interactions. Although drug interactions with antiepileptic drugs (AEDs) are rare during absorption, such interactions can be important in some cases. Interactions involving the distribution of drugs are difficult to ascertain. Distribution of AEDs from the blood compartment to the brain is very necessary for a successful therapeutic outcome. Metabolism is the most important mechanism of elimination and accounts for the majority of clinically relevant drug interactions with AEDs. By far the most important pharmacokinetic interactions with AEDs are those which are related to induction or inhibition of drug metabolism. Although drug interactions affecting renal excretion are rare with AEDs, AEDs that undergo extensive renal elimination in unchanged form may be susceptible to interactions affecting the excretion process.