The review addresses the problem of the diagnosis of Lennox—Gastaut syndrome, a severe epileptic encephalopathy. Despite the presence of a vivid clinical and encephalographic picture, classical Lennox—Gastaut syndrome, which meets all of its diagnostic criteria, is quite rare. Many authors believe that the diagnosis of the syndrome is possible if the patient has tonic seizures and typical ictal and interictal patterns on the electroencephalogram (EEG). The diagnosis of the syndrome is considered probable if there are typical EEG patterns of wakefulness and sleep, but no tonic seizures are recorded. Diagnosis of the syndrome is complicated by its polyetiology (clinical and EEG manifestations can vary significantly), the evolution of seizure types and EEG characteristics as the patient matures, the presence of other epileptic syndromes similar to Lennox—Gastaut syndrome.