Human prion diseases (PrD) like Creutzfeldt-Jakob disease (CJD) include sporadic, acquired and familial neurodegenerative disorders. The central events in the neuropathological process of PrDs are severe neuronal loss, spongiform change and accumulation of abnormal prion protein (PrPSc). The latter is a conformational variant of the host-encoded cellular PrP (PrPC), a copper-binding protein. The physiological role of PrPC is debated. Definitive diagnosis of PrD is based on post mortem demonstration of PrPSc by immunohistochemistry or Western blot. Mutations in the PrP gene (PRNP), the polymorphic site at codon 129, and the molecular characteristic of protease resistant PrP influence the phenotype. Clinical symptoms, cranial MRI scan, EEG and investigation of 14-3-3 protein in cerebrospinal fluid (CSF) suggest a diagnosis of probable CJD. Variant CJD, related to bovine spongiform encephalopathy, shows a different clinical course, symmetrical high intensity MRI signal in the pulvinar, presence of PrPSc in tonsil biopsy tissue, and a lower sensitivity of CSF 14-3-3 protein compared to sporadic CJD. Future possibilities in diagnosis of PrDs include either the demonstration of PrPSc in body fluids or disease associated changes in laboratory variables or gene expression.