Studies on gastric digestion during 1820-1840 led to the discovery of pepsin as the agent which, in the presence of stomach acid, causes the dissolution of nutrients such as meat or coagulated egg white. Soon afterward it was shown that these protein nutrients were cleaved by pepsin to diffusible products named peptones. Efforts to isolate and purify pepsin were spurred by its widespread adoption for the treatment of digestive disorders, and highly active preparations were available by the end of the nineteenth century. There was uncertainty, however, as to the chemical nature of pepsin, for some preparations exhibited the properties of proteins while other preparations failed to do so. The question was not settled until after 1930, when Northrop crystallized swine pepsin and provided convincing evidence for its identity as a protein. The availability of this purified pepsin during the 1930s also led to the discovery of the first synthetic peptide substrates for pepsin, thus providing needed evidence for the peptide structure of native proteins, a matter of debate at that time. After 1945, with the introduction of new separation methods, notably chromatography and electrophoresis, and the availability of specific proteinases, the amino acid sequences of many proteins, including pepsin and its precursor pepsinogen, were determined. Moreover, treatment of pepsin with chemical reagents indicated the participation in the catalytic mechanism of two aspartyl units widely separated in the linear sequence. Studies on the kinetics of pepsin action on long chain synthetic peptides suggested that the catalytic site was an extended structure. Similar properties were found for other "aspartyl proteinases," such as chymosin (used in cheese making), some intracellular proteinases (cathepsins), and plant proteinases. After 1975, the three-dimensional structures of pepsin and many of its relatives were determined by means of x-ray diffraction techniques, greatly extending our insight into the mechanism of the catalytic action of these enzymes. That knowledge has led to the design of new inhibitors of aspartyl proteinases, which are participants in the maturation of human immunodeficiency virus and in the generation of Alzheimer's disease.