Magnetic islands are a ubiquitous feature of magnetically confined plasmas. They arise as the result of plasma instabilities as well as externally imposed symmetry-breaking perturbations. In the core, effective suppression techniques have been developed. Even thin islands, however, are observed to have nonlocal effects on the profiles of rotation and current. This has stimulated interest in using magnetic islands to control plasma transport, particularly in the edge. They are also of interest as a tool to improve our understanding of microscopic plasma dynamics.