The ketogenic diet, which consists of reduced carbohydrate intake and increased fat intake, is a recognized treatment option for children with intractable epilepsy. This diet is now receiving renewed interest from physicians and researchers because of its potential therapeutic effect in other diseases, such as neurodegenerative diseases, metabolic syndrome or cancer. Since cancer is one of the major public health challenges, complementary approaches to improve the efficacy of standard anti-cancer therapies are the subject of much research. This article reviews the place of the ketogenic diet as a complementary therapy in cancer, the scientific evidence and possible practical aspects of such an approach.