Ketogenic diet therapy (KDT) is a treatment option, which in recent years has been used increasingly more for therapy resistant epilepsies in children and adults. The KDT is the therapy of choice for two diseases of cerebral energy metabolism, the glucose transporter 1 deficiency and the pyruvate dehydrogenase deficiency. In addition to the known indications, such as early infantile encephalopathies, severe epilepsy in childhood, adolescence and adulthood, KDT has also become established for the treatment of super-refractory status epilepticus. The list of indications is also expanding into other fields of neurology and internal medicine.There is a syndrome-specific efficacy of KDT in seizure disorders. The efficacy of KDT is assessed after 3 months. If ineffective, the KDT can be rapidly stopped, otherwise it is continued for 2 years. Afterwards, attempts to stop the therapy can be made, but it can be continued for many years if necessary.Besides the classical ketogenic diet, various other practical forms of KDT have been developed, which are easier to implement in the routine: the modified Atkins diet (MAD), the medium-chain triglyceride (MCT) diet and the low glycemic index treatment (LGIT). The MAD has become the most established form worldwide. All forms of KDT represent an unbalanced nutrition. Therefore, a substitution of vitamins and minerals is mandatory. For the implementation of a KDT and the regular control of the efficacy and possible, particularly renal (kidney stones) side effects, a keto-team consisting of the treating physician, a dietician and nursing personnel is needed.