Pharmacogenetics and pharmacogenomics are fields which show how the genetic make-up of an individual can influence drugs effects. Genetic polymorphisms can influence antiepileptic drug (AED) responses and, during polytherapy, their interaction profile by influencing metabolism, central nervous system penetration, pharmacodynamics and adverse events. This chapter considers the evidence for each of these and reviews the uses and pitfalls of genetic screening. The point of action for AEDs is the brain, and so AEDs have to be able to cross the blood-brain barrier. GABAA receptors are the target for a number of AEDs since alterations in GABAA receptor-mediated transmission have been implicated in the pathogenesis of epilepsy. The predictive value of a pharmacogenetic test can be viewed from a genetic epidemiological perspective. Clinical and cost effectiveness of pharmacogenetic tests may need to be established in prospective randomized trials and their use may require new professional standards of testing and test interpretation.