Considering the current interest in cellular regulation and intracellular signalling systems, it is surprising that the contribution of ADP-ribosylation reactions to the modulation of a variety of specific cell processes, in parallel with other post-translational modifications such as phosphorylation, has not been generally recognized. While it is not feasible to cover all aspects of ADP-ribosylation, the thirty-one articles contained in this volume provide a valuable overview of recent progress in the field within the context of cell control mechanisms. For the convenience of the reader, the various topics have been grouped into several sections: (a) poly(ADP-ribosyl)ation; (b) mono-ADP-ribosylation; (c) toxin mono-ADP-ribosylation; (d) inhibitors and activators; (e) protein modification with ADP-ribose and its analogues; and (f) non-modification forms of ADP-ribose. The contents of the individual chapters reflect the ideas of the contributors, many of whom have spent their careers attempting to resolve the biological functions of ADP-ribosylation. We hope that this publication will serve as a useful reference for those investigators that are new to the area as well as those who are actively studying ADP-ribosylation.