The ketogenic diet (KD) has been used to treat children with epilepsy who are resistant to antiepileptic drugs (AEDs) since the 1920s, and has undergone a resurgence in popularity over the last 15 years Its use in adolescents and adults has been more restrained. During the past few decades, more liberal regimens have emerged that may seem more attractive to older people while still proving effective, often independent of ketone levels. The KD and its variants may lead to similar reductions in seizure frequency in adolescents and adults as seen in children, although studies are limited and of poor quality. A total of only 122 adults and 82 adolescents have been included in open-label studies on the KD, and only 56 adults and 10 adolescents on the Modified Atkins Diet. Side effects appear similar to those encountered in children. Noncompliance may be higher in adolescents and adults than in children, but the main reason for discontinuation is lack of efficacy. A better understanding of the mechanisms underlying the effects of the KD might allow the same treatment effects to be achieved using novel, better-tolerated, nondietary approaches.