What are the terrestrial effects of solar activity and the solar activity cycle? The modern term used for solar-terrestrial relations is "Space Weather". This term describes all external effects on the space environment of the Earth and the Earth's atmosphere. The main driver for space weather is our Sun. Explosive events on the Sun that are modulated by the solar activity cycle lead to enhanced particle emission and short wavelength radiation. This affects satellites: for example surface charging and enhanced drag forces on satellites in low Earth orbit can cause satellite crashes etc. Enhanced radiation also poses a problem for astronauts, especially for extravehicular activities. Another source of space weather effects is space debris and micrometeoroids. Since the Sun is the main source of space weather effects, the first part of the book is devoted to a general introduction to the physics of the Sun. A better understanding of the phenomena underlying solar activity is also important for prediction of solar outbursts and thus for establishing alert systems for space missions and telecommunication systems.