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