The frequency of reports concemmg the interface of biological reco- tion elements to signal transduction technologies has risen dramatically over the last decade. Because any one of a wide variety of biological recognition elements (e. g., antibodies, receptors, DNA, microorganisms, or enzymes) can theoretically be interfaced with any one of a wide variety of signal transducers (e. g., optical, electrochemical, thermal, or acoustic), the potential range of devices and techniques can be bewildering. The purpose of this volume and the previous volume in this series is to provide a basic reference and startmg point for investigators in academics, mdustry, and government to begin or expand their biosensors research. This volume, Methods in Biotechnology vol. 7: Affinity Biosensors: Techniques and Protocols, describes a variety of classical and emerging transduction technologies that have been interfaced to bioaffinity elements (e. g., antibodies and receptors). Some of the reasons for the expansion in the use of affinity-based biosensors include both advances in signal transduction technologies (e. g., fiber optics, microelectromcs, and microfabrication) and the availability of bioafflmty elements. More specifically, with respect to biological recognttion elements, commercially and noncommercially produced antibodies directed toward a variety of analytes have become widely available. In addition, te- niques for the purification and stabilization of receptors have also significantly improved. As a result of these recent advances in the field, biosensors research and development projects are being pursued by mvestigators from a wide range of disciplines.