This book is the first comprehensive review and synthesis of our understanding of the origin, evolution, and effects of magnetic fields in stars that, like the Sun, have convective envelopes immediately below their photospheres. The resulting magnetic activity includes a variety of phenomena that include starspots, nonradiatively heated outer atmospheres, activity cycles, the deceleration of rotation rates, and – in close binaries – even stellar cannibalism. Our aim is to relate the magnetohydrodynamic processes in the various domains of stellar atmospheres to processes in the interior. We do so by exploiting the complementarity of solar studies, with their wealth of observational detail, and stellar studies, which allow us to study the evolutionary history of activity and the dependence of activity on fundamental parameters such as stellar mass, age, and chemical composition. We focus on observational studies and their immediate interpretation, in which results from theoretical studies and numerical simulations are included. We do not dwell on instrumentation and details in the data analysis, although we do try to bring out the scope and limitations of key observational methods.

This book is intended for astrophysicists who are seeking an introduction to the physics of magnetic activity of the Sun and of other cool stars, and for students at the graduate level.