The Sun, as our nearest star, is of enormous importance for life on Earth, providing the warm radiation and light which allowed complex life to evolve. The Sun plays a key role in influencing our climate, while solar storms and high-energy events can threaten our communication infrastructure and satellites. This Very Short Introduction explores what we know about the Sun–its physics, its structure, origins, and future evolution. Philip Judge explains some of the remaining puzzles about the Sun that still confound us, using elementary physics and mathematical concepts. Why does the Sun form spots? Why does it flare? As he shows, these and other nagging difficulties relate to the Sun’s continually variable magnetism, which converts an otherwise dull star into a machine for flooding interplanetary space with variable radiation, high-energy particles, and magnetic ejections. Throughout, Judge highlights the many reasons that the Sun is important and why scientists engage in solar research.