In 1858, Richard Carrington wrote, “Our knowledge of the Sun's action is but fragmentary, and the publication of speculations on the nature of his spots would be a very precarious venture.” Fifty years later, George Ellery Hale's discovery of the magnetic field in a sunspot ushered in the modern era of research into solar, stellar and cosmical magnetic fields. This book, coincidentally, marks the hundredth anniversary of his discovery. The past century has seen enormous and rapidly accelerating progress in our understanding not only of sunspots but also of starspots and the whole solar–stellar connection. Our purpose here is to bring these advances together and to offer a unified account of sunspots and starspots in the context of solar and stellar magnetic activity.

Our own collaboration goes back more than 40 years, to the academic year 1966–67 when JHT was a NATO postdoctoral fellow in the Department of Applied Mathematics and Theoretical Physics at Cambridge, where NOW was a recently appointed lecturer. In 1991 we organized a NATO Advanced Research Workshop on Sunspots: Theory and Observations, which produced an edited volume (Thomas and Weiss 1992a) designed to serve as a monograph on the subject. Progress on sunspots has been very rapid since then, especially in high-resolution observations (both ground-based and from space) and in numerical modelling; meanwhile, with new techniques such as Doppler and Zeeman–Doppler imaging, the study of starspots (treated briefly in the 1992 volume) has emerged as a fully fledged subject of its own. Hence it seems to us that the time has come for a new, comprehensive book on sunspots and starspots that emphasizes recent developments.