A first edition of this book appeared in Russian language in 1983. Since those days the interest in proteolytic enzymes has acquired a new impulse due to advances in genetic engineering techniques, which facilitated structural and func tional studies of the enzymes. Much more data on a crucial role of proteases in biological processes in norm and pathology are available now. Information that appeared in the past 8 years prompted a renewal of the book and furnishing a supplement especially made for the English edition. The book retains the presentation of the previous edition and yet enlarges the scope to cover the methods for site-directed mutagenesis of amide hydrolases, catalyti cally active antibodies, etc. I have tried to preserve the book as a means of reference for the reader. A list of enzymes has been expanded and the biblio graphy up to 1990 enlarged by half as much. I am grateful to the translators T. E. Chernichko, N. 1. Loboda and my co worker Dr. S. L. Alexandrov for their assistance in the preparation ofthis edition and I feel happy that my book is now available for enzymologists abroad.