Severe myoclonic epilepsy of infancy (SMEI) is a complex form of epilepsy that was first described in France in 1978. Because the myoclonic component of this epilepsy is not always present and because some variability has been observed in the symptomatology, the name was changed to Dravet syndrome in 1989. The genetic aetiology of this epilepsy was discovered in 2001, and since then numerous studies have contributed to a better knowledge of the disease. Around 70% of affected patients are carriers of a mutation on the alpha subunit of the "SCN1A" gene. An accurate analysis of the clinical features leads to the distinction between typical and atypical forms, both with the same unfavourable prognosis and the same genetic background. However, many studies are being conducted in order to establish correlations between phenotypes and genotypes, and to understand the factors underlying the cognitive impairment of the affected patients.