Since the discovery of high-temperature superconductivity considerable progress has been made in research and development of superconductive electronic devices. This fact is reflected by the large variety of contributions to the 4th International Conference on Superconducting and Quantum Effect Devices and their Applications - SQUID'91. This comprehensive proceedings volume covers Josephson junctions, SQUIDs, radiation detectors, flux-flow transitors. The main applications are contained in the chapters on biomagnetic instrumentation, metrologym and rf-applications. The purpose of this volume is to provide an up-to-date reference book on thestatus of this rapidly changing and promising field of electronic device research on the basis of low- and high-temperature materials.