The importance of microelectrodes is widely recognised and interest in their application in diverse areas of research has been increasing over the past ten years. In fact, several meetings organized by the International Society of Electrochemistry, The American Chemical Society and The U. S. Electrochemical Society have analysed various aspects of their theory and applications. For this reason it seemed that the time had arrived when scientists from around the world, actively concerned with research in the area of microelectrodes, should meet, exchange ideas and assess the direction of future developments. Furthermore, it seemed appropriate that this meeting should be held as a NATO Advanced Study Institute, so that students and young scientists with research interests in microelectrodes would have the opportunity to interact with experts in the field, establish future collaboration and, hopefully, catalyse new developments in the area. The meeting was held in Alvor, Portugal, in May 1990. This book compiles the lectures delivered in the Institute. It reviews the most important aspects of microelectrodes and points out directions for future research in this field. Several contributions discuss recent developments in theoretical aspects such as the properties of various geometries and computational procedures for solving the equations describing the coupling of mass transport to microelectrodes with heterogeneous electron tranfer and homogeneous chemistry. The materials and methods available for microelectrodes manufacture are presented in some detail. Both steady state and transient techniques are covered and the interaction of theory with experiment is discussed.