Many neurodegenerative diseases—including Creutzfeldt-Jakob disease, Alzheimer's disease (AD), Parkinson's disease, and amyotrophic lateral sclerosis (ALS)—share two remarkable characteristics. The first is that more than 80% of cases are sporadic. The second is that although many of the disease-specific mutant proteins are expressed in embryogenesis, the inherited forms of these neurodegenerative diseases are late-onset. This suggests that some event occurs with aging that renders a disease-specific protein pathogenic. More than 20 years ago, I argued that this event involves a stochastic refolding of the etiologic protein into a misfolded infectious state known as a prion. In the past decade, there has been renewed interest in the possibility that the proteins causing neurodegeneration are all prions, which would profoundly influence the development of diagnostics and effective therapies.