The intricate interactions between transition metals and ligands are at the heart of a wide variety of chemical, physical and biological phenomena. Understanding these interactions provides a challenge of the first order, cutting across many fields of modern science and impinging on numerous areas of application. The fascinating behavior of these systems has attracted the attention of experimentalists and theorists alike. In this book are collected the main lectures of the NATO ASI on "Metal-Ligand Interactions in Chemistry, Physics and Biology" held in Cetraro, Italy, in September, 1998. This event followed two previous NATO ASI held also in Cetraro in 1991 and 1994 dedicated to "Metal- Ligand Interactions: from Atoms, to Clusters, to Surfaces" and to "Metal- Ligand Interactions: Structure and Reactivity", respectively. The increasing number of participants (115) and of requests to attend (more than 400) confirms the importance and timeliness of this sequence of ASI's as well as the high level of interest for the themes treated. The lectures were organized on the basis of the following topics: -clusters and surfaces -catalysis -inorganic complexes -bioinorganic systems -new experimental techniques -theoretical methodologies and were given by well known leading experts such as: Ivano Bertini, David A. Case, Bernard Coq, Gernot Frenking, Hans J. Freund, Francesc Illas, Jacek Lipkowski, Julius Jellinek, Nino Russo, Dennis R. Vll viii Salahub, Karlheinz Schwarz, Friedrich Siebert, Benoit Simard, Edward I. Solomon and Michael Zemer.