Described in the early 1920s in Germany, Creutzfeldt-Jakob disease now comprises 4 entities: 1. familial forms are linked to mutations of the gene coding for prion protein (about 8% of the cases); 2. iatrogenic forms are due to interventions using contaminated material or human extracts (about 5% of the cases); 3. sporadic forms represent the majority of the cases (about 87%) without any established explanation; 4. recently described new variant, related to bovine spongiform encephalopathy, remains quite rare (41 known cases). In all its forms, the disease is constantly mortal after a usually short course. Major signs are rapidly evolving; dementia with myoclonus, ataxia, and electroencephalographic abnormalities (periodic activity). Formal diagnosis is histologic.