Of all the recent discoveries in biotechnology, that of biosensor is one of those which has seen an exponential expansion over the last few years. This evolution corresponds with the increasing need for measuring devices that can follow continuously changing biological processes. Biosensors can meet this need provided that their signals include all the information necessary for an understanding of the process, especially concerning the nature and concentration of the species present in the sample medium. It is well known that sensors form the basis of all instrumental analysis systems, but they also represent the limiting factors of such systems. In this book, we restrict ourselves to the description and study of sensors, leaving aside the different aspects of signal and data treatment. We believe, however, that it is important to stress the multifaceted character of biosensors, and the applications and economic factors which follow. Biosensor construction is essentially based on the immobilization of a bioreceptor on the corresponding transducer. The reader will find that there are a large variety of techniques for immobilizing enzymes, cofactors and mediators, and even microorganisms, immunoagents, e now commercially available. Other types of tissues, and organelles. A large part of this book is devoted to enzyme biosensors are discussed, with regard to both the principles of their sensors, which is hardly surprising considering that they have been operation, and their construction.