Alzheimer’s disease and Creutzfeldt–Jakob disease are the best-known examples of a group of diseases known as the amyloidoses. They are char-acterized by the extracellular deposition of toxic, insoluble amyloid fibrils. Knowledge of the structure of these fibrils is essential for understanding the process of pathology of the amyloidoses and for the rational design of drugs to inhibit or reverse amyloid formation.