The demand for selective organic reactions is growing more acute everyday. Indeed, greater product selectivity has an important impact on energy and resource utilization, in terms of reduced process energy requirements for product separation and purification, in terms of low-value by-products, and in terms of environmental acceptance and compatibility. Moreover, more and more chemicals, especially pharmaceuticals, have to be sold in an optically active form. The search for selectivity constitutes a tremendous challenge for the chemists. In the last two decades, homogeneous transition metal based catalysis has emerged as one of the most promising tools for obtaining selectivity. In connection with developments in this area, this book contains updated and expanded versions of most of the lectures presented at a Cornett course held in Trieste (Italy) in 1989 and sponsored by the European Community. A primary aim is to cultivate a deeper understanding of the parameters that govern the selectivities and stimulate a wider utilization of transition metal based catalysis in organic synthesis. All aspects of selectivity, chemo-, regio-, stereo- and enantioselectivity are considered and illustrated by applications in various fields or organic synthesis. The impact of catalysis in oxydation, reduction, carbonylation reactions, carbene chemistry, in Ni and Pd promoted dimerizations, oligomerizations as well as fonctionalisations is stressed, quite often with special emphasis laid on reaction mechanisms. In this aspect, the last chapter examplifies the interest of high pressure NMR and IR when investigating the nature of reaction intermediates in homogeneous reactions.