A great deal of effort during the past 27 years has been devoted to defining the chemical nature of prions, the infectious agents responsible for transmissible spongiform encepha-lopathies. Prion diseases are fatal neurodegenerative disorders that can arise spontane-ously, be inherited, or be acquired by infection in mammals. They are unique not only in terms of their biological features but also in terms of their impact on public health. It has been hypothesized that in addition to Creutzfeldt - Jakob disease (CJD) in humans and Bovine Spongiform Encephalopathy (BSE) in animals, prions may also play a role in sev-eral other neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and frontotemporal dementia; however, the precise mecha-nism underlying prion-mediated neurodegeneration still remains elusive. In this review, we outline the physico-chemical characteristics of prions and their impact on human and animal health.