This book summarizes the history and present status and applications of Josephson junctions. These devices are leading elements in superconducting electronics and provide state-of-the-art performance in detection of small magnetic fields and currents, in several digital computing methods, and in medical diagnostic devices and now provide voltage standards used worldwide. Astronomical infrared (IR) telescopes, including the South Pole Telescope, use these junctions in combinations called superconducting quantum interference devices (SQUIDs).