Superconductors are materials that conduct electricity with no resistance. This means that, unlike the more familiar conductors such as copper or steel, a superconductor can carry a current indefinitely without losing any energy. They also have several other very important properties, such as the fact that no magnetic field can exist within a superconductor. Superconductors already have drastically changed the world of medicine with the advent of MRI machines, which have meant a reduction in exploratory surgery. Power utilities, electronics companies, the military, transportation, and theoretical physics have all benefited strongly from the discovery of these materials. This book deals with various aspects of superconductivity, both theoretical and experimental. Some of the technological applications of superconductivity include: the production of sensitive magnetometers based on SQUIDs; fast digital circuits (including those based on Josephson junctions and rapid single flux quantum technology); powerful superconducting electromagnets used in maglev trains; Magnetic Resonance Imaging (MRI) and Nuclear magnetic resonance (NMR) machines, magnetic confinement fusion reactors, and the beam-steering and focusing magnets used in particle accelerators; low-loss power cables, etc.,