This Handbook of Proteolytic Enzymes is a massive undertaking by Alan Barrett, Neil Rawlings, and Fred Woessner, the book's three editors. Not only was the multi-author Handbook designed and edited by them, but also they have made significant contributions in writing major sections of the book. The Handbook consists of 1666 pages, 569 chapters organized into four major catalytic peptidase classes: serine (threonine), cysteine, aspartic, and metallopeptidases. An additional category deals with presently unclassified peptidases. Within each of the above mechanistic categories, the peptidases are grouped into clans and families, the members of which derive from a common evolutionary ancestor. The members of a family have statistically significant similarities in the amino acid sequences of the peptidase portion of the molecules. That the individual families of a clan are derived from a common evolutionary ancestor is usually not detectable at the level of amino acid sequence. This classification scheme was the brain-child of Rawlings and Barrett (1993) and has gained wide acceptance as the major system to unify the previously rather arbitrary nomenclature of proteolytic enzymes.