Most stars show some degree of magnetic activity, from the familiar variations in the Sun's magnetic field, which coincide with the sun-spot cycle, to the enormous magnetic fields created by rotating neutron stars. Magnetic fields are also a potential key to understanding the formation of new stars and the behavior of galactic nuclei. This book by one of the leading figures in stellar magnetism provides an authoritative survey of this rapidly developing field. Based on a lifetime of research, the book places stellar magnetism in a broad astronomical scope and provides a thorough, well-argued treatment of current work. It covers the key topics, discussing the relevant mathematics in detail and including numerous references, and many of the topics, particularly accretion discs, dynamos, and winds, are equally important to the study of galaxies and galactic nuclei.