In this volume of Methods in Enzymology and its companion Volume 244, the chapters on specific methods, enzymes, and inhibitors are organized within the rational framework of the new systems for classificationand nomenclature. A wide variety of specificities of peptide bond hydrolysis are represented in each set of peptidases, together with an equally wide range of biological functions.Key Features• Aspartic peptidases• Metallopeptidases• New information on classification of proteolytic enzymes• Medical implications of research in this area• Biotechnological uses of these enzymes