Since the discovery of high-temperature superconductivityconsiderable progress has been made in research anddevelopment of superconductive electronic devices. This factis reflected by the large variety of contributions to the4th International Conference on Superconducting and QuantumEffect Devices and their Applications - SQUID'91. Thiscomprehensive proceedings volume covers Josephson junctions,SQUIDs, radiation detectors, flux-flow transitors.. The mainapplications are contained in the chapters on biomagneticinstrumentation, metrologym and rf-applications. The purposeof this volume is to provide an up-to-date reference book onthestatus of this rapidly changing and promising field ofelectronic device research on the basis of low- andhigh-temperature materials.