The ketogenic diet (KD) is an effective nonpharmacological treatment used as an alternative not only for children but also for adults in the management of refractory epilepsy. Recently, modifications of the original KD, such as the Atkins diet and a low-glycemic-index diet, have been developed. The availability of these types of diets suggests that the KD could be offered earlier in the management of refractory epilepsy. Nevertheless, these diets may not be as effective as the KD. The KD is the treatment of choice for two distinct disorders of brain energy metabolism: glucose transporter protein (GLUT-1) deficiency syndrome and pyruvate dehydrogenase deficiency (PDHD) syndrome. The KD is absolutely contraindicated in primary carnitine deficiency, carnitine palmitoyltransferase I or II deficiency, carnitine translocase deficiency, β-oxidation defects (long-chain acyl dehydrogenase deficiency, medium-chain acyl dehydrogenase deficiency, short-chain acyl dehydrogenase deficiency, long-chain 3-hydroxyacyl-coA deficiency, medium-chain 3-hydroxyacyl-coA deficiency), pyruvate carboxylase deficiency, and porphyria. The diet may also be beneficial for seizure control in specific epileptic syndromes such as epilepsy with myoclonic-astatic seizures, West syndrome, Lennox-Gastaut syndrome, and Dravet syndrome. After stopping the KD, seizures recur in a few patients with cerebral lesions and electroencephalogram abnormalities; however, long-term health implications should be investigated.