Even a casual search of recent literature would show that the involvement of transglutaminases in a great variety of biological reactions has captured the imagination of many investigators. The present volume of Molecular and Cellular Biochemistry is a reflection of this growing interest. Transglutaminases catalyze the post-translational modification of proteinbound glutamine residues which either causes protein cross-linking by 'Y-glutamyl-e-Iysine peptide bridges or covalent incorporation of small molecular weight amines. These reactions could have far-reaching effects on altering the physical states and biochemical behaviors of the extra-cellular or intra-cellular systems. Examples could be drawn from clotting in blood or in seminal fluid, from keratinization, from cell aging, or from cell activation. Within the framework of the definition, the transglutaminase name denotes not just a single enzyme but a large group of enzymes with distinct characteristics. Multiple forms are often found in a given tissue or cell type, but, as yet, we do not know the reasons for this diversity. Editors: Laszlo Lorand, Ph.D. Victor A. Najjar, M.D. Molecular and Cellular Biochemistry 58, 9-35 (1984). © 1984, Martinus Nijhoff Publishers, Boston. Transglutaminases Laszlo Lorand and Sylvia M. Conrad Department of Biochemistry, Molecular and Cell Biology, Northwestern University, Evanston, IL 60201, U.S.A.