In this chapter, the internal structure of the Sun and its atmospheric layers are described. The solar magnetic fields are generated in the convection zone by magnetic induction in dynamo processes. In the solar atmosphere, magnetic reconnection processes continuously trigger restructurings of the magnetic fields, which, above all, change over the course of an approximately 11-year cycle of activity. The formation and evolution processes of sunspots, flare regions, spicules in the magnetic network, magnetic field arcs, gas clouds, eruptions, and mass ejections are discussed in detail and vividly illustrated. Sporadic eruptions of the magnetic field arcs and prominences in the corona give rise to ejections of matter far out into the heliosphere. Such solar storms play a crucial role in determining space weather. Important terms from magnetohydrodynamics are explained and the basic principles of solar dynamo and reconnection processes are presented in more detail.