The physics and applications of the Josephson effect are reviewed in detail. A brief introduction to superconductivity and tunneling is provided. The configuration most fully discussed is the oxide junction. However, properties of bridges, point-contacts, and solder-blobs are also described. Among the applications reviewed are magnetometers, infrared detectors, microwave generators and mixers, voltmeters, computer logic and memory devices, relaxation oscillators, thermometers, and spectrometers.