In the past ten years, a number of proceedings of symposia on the structure and function of proteolytic enzymes have been pub- lished. Their coverage of acid proteases has been limited, mainly due to the lack of significant new information on the structure of these enzymes. In the last four years, however, the primary and tertiary structures of a number of acid proteases have been deter- mined, prompting the need to discuss the meanings of the old data and the possibilities for new experimentations. It was for this purpose that the "Conference on Acid Proteases: Structure, Function, and Biology" was organized. It took place at the University of Oklahoma on November 21-24, 1976. This book is a collection of the main lectures delivered at the Conference. Acid Proteases, by definition refers to a group of proteases having an optimal p H in acidic solutions. The classic examples are pepsin and chymosin. Some catalytic features are obviously shared by these proteases, most notably, their inhibition by pepstatin. The use of active center-directed inactivators such as diazoacetyl- norleucine methyl ester and 1, 2-epoxy-3-(p-nitrophenoxy)propane has shown that two catalytic aspartyl residues are present in most of these enzymes. These apparent cornmon features have prompted the suggestion by several investigators to name this group of enzymes "aspartyl proteases" or "carboxyl proteases".