Prions are self-perpetuating aggregated fibrous proteins that associated with fatal transmissible spongiform encephalopathies (TSEs) in mammals (including humans), and manifest themselves as non-Mendelian heritable elements in yeast and other fungi. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrP^C) of the host. PrP^C is necessary for prion replication and for prion-induced neurodegeneration, yet causes of neuronal injury and death are still poorly understood. Here we view of the prion concept, models describing of the replication and transport of prions particles, structural features and functions of the cellular PrP, the prion strain phenomenon, current developments in diagnostics of prion and potential antiprion therapies. Finally, we discuss how prion-like mechanisms may apply to other protein aggregation diseases.