Book excerpt: Handbook of Proteolytic Enzymes, Second Edition, Volume 1: Aspartic and Metallo Peptidases is a compilation of numerous progressive research studies on proteolytic enzymes. This edition is organized into two main sections encompassing 328 chapters. This handbook is organized around a system for the classification of peptidases, which is a hierarchical one built on the concepts of catalytic type, clan, family and peptidase. The concept of catalytic type of a peptidase depends upon the chemical nature of the groups responsible for catalysis. The recognized catalytic types are aspartic, cysteine, metallo, serine, threonine, and the unclassified enzymes, while clans and families are groups of homologous peptidases. Homology at the level of a family of peptidases is shown by statistically significant relationship in amino acid sequence to a representative member called the type example, or to another member of the family that has already been shown to be related to the type example. Each chapter discusses the history, activity, specificity, structural chemistry, preparation, and biological aspects of the enzyme. This book will prove useful to enzyme chemists and researchers.