The ketogenic diet is a high-fat, low-protein, low-carbohydrate diet that has been employed as a non-pharmacologic therapy for refractory epilepsy. Several multicenter and two randomized studies have demonstrated the efficacy of the ketogenic diet and the modified Atkins diet for children and adolescent with pharmacoresitant epilepsy. In order to facilitate patient tolerability and palatability, the diet protocols are gradually modified including changes in ratios of the fat versus non-fat components and the initiation of the diet with or without fasting. The modified Atkins diet is now used as an alternative diet. A randomized trial establishing the efficacy of the modified Atkins diet is now available. More recently, the low glycemic index diet seems to be used successfully for pharmacoresistant epilepsy but there are currently only open studies. Looking at the clinical efficacy of dietary treatments, the studies usually report a greater than 50% reduction in seizure frequency in about half of patients at 3 months under diet. Most of the patients who are responders to the ketogenic diet exhibited a decrease in seizure frequency within two months of treatment onset. Efficacy of the ketogenic diet has also been reported for teenager and adult patients. Dietary treatment of epilepsy should not be considered as a last chance treatment. It can be used during the investigation for epilepsy surgery even in case of structural abnormalities. In some epilepsy syndromes such as infantile spasms, myoclonic-astatic epilepsy and refractory status epilepticus, an early use seems helpful. The exact underlying mechanisms are unknown and remain a topic of active research.