Correct classification of seizure type and epilepsy syndrome are important first steps in the management of patients with epilepsy, providing critical information for choice of diagnostic tests, selection of optimal treatment, and determination of prognosis. The two currently accepted classifications are the International Classification of Epileptic Seizures and the International Classification of Epilepsies and Epileptic Syndromes. Epileptic seizures are divided into focal seizures, which begin in a localized region of one hemisphere, and generalized seizures, which involve both hemispheres at onset. Epilepsy syndromes are constellations of features such as seizure type, age of onset, etiology, interictal deficits, and EEG and neuroimaging findings. Epilepsy syndromes are divided into syndromes with focal seizures, syndromes with generalized seizures, undetermined syndromes, and special syndromes. They are further subdivided by etiology. Idiopathic syndromes have a presumed genetic etiology, while symptomatic epilepsies have a clearly identified cause. Probably symptomatic syndromes are those in which an etiology is suspected, but the exact cause cannot be determined. Over the past 6 years, the International League Against Epilepsy (ILAE) Task Force on Classification and Terminology has proposed important modifications for these classification systems. In addition, a new diagnostic scheme for patients with epilepsy provides structure in the form of five diagnostic axes: ictal phenomenology, seizure type, epilepsy syndrome, etiology, and impairment. This chapter focuses on the current classifications and proposed diagnostic axes in adolescents and adults. Proposed modifications that reflect important changes in classification philosophy will also be discussed.