Human diseases characterized by insoluble extracellular deposits of proteins have been recognized for almost two centuries. Such amyloidoses were once thought to represent arcane secondary phenomena of questionable pathogenic significance. But it is has now become clear that many different proteins can misfold and form extracellular or intracellular aggregates that initiate profound cellular dysfunction. Particularly challenging examples of such disorders occur in the post-mitotic environment of the neuron and include Alzheimer's and Parkinson's diseases. Understanding some of the principles of protein folding has helped to explain how such diseases arise, with attendant therapeutic insights.