This book provides an introduction to our current knowledge of stellar magnetic activity. It is intended for graduate or advanced undergraduate students or researchers new to the field. Starting with the Sun, it extends to stars of all masses and ages. Topics include the production and measurement of the fields themselves, their effects on the stellar atmospheres, the diagnostics we use to understand them, and the effects of the magnetic activity on the star and its surrounds over time. This field is rapidly advancing, and the book will serve as an initial resource for those new to the area, or as an overview for those already involved who want a wider or historical perspective, along with suggestions as to which directions that new research might take.Key Features: Provides simple physical explanations of the diagnostics used to understand stellar activityTakes a historical approach to the subject, including a look to the futureRefers to milestone articles, with an explanation of their importanceContains numerous diagrams, images, and videosUses simple language to convey the science