The ketogenic diet (KD) has been used successfully in a variety of epilepsy syndromes. This includes syndromes with multiple etiologies, including Lennox-Gastaut syndrome and infantile spasms; developmental syndromes of unknown etiology, such as Landau-Kleffner syndrome; and idiopathic epilepsies, such as myoclonic-astatic (Doose) epilepsy. It also includes syndromes where genetics play a major role, such as Dravet syndrome, tuberous sclerosis, and Rett syndrome. Study of the KD in humans and animals harboring various genetic mutations may yield insights into the diet's mechanisms. Comparison of the diet's effectiveness with other treatments in specific syndromes may be another useful tool for mechanistic studies. The diet's utility in epilepsy syndromes of various etiologies and in some neurodegenerative disorders suggests it may have multiple mechanisms of action.