Written from both an observational and historical perspective, this book is the first to provide a comprehensive combined account of sunspots and starspots. It begins with a substantial historical introduction, then explores the intricate structure of a sunspot's magnetic field and the prevalence of polar spots on stars. Additional topics covered in depth include solar and stellar magnetic activity, dynamo models of magnetic cycles, and the influence of solar variability on the Earth's magnetosphere and climate. It is a valuable reference for graduate students and specialists in solar and stellar physics, astronomers, geophysicists, space physicists and experts in fluid dynamics and plasma physics.