The role of solar variability in climate variability and change has been debated for a long time. Now, new results from various space experiments monitoring the radiative and particle emissions from the Sun together with detailed studies of their terrestrial impacts have opened an exciting new era in both solar and atmospheric physics. Being so close, the Sun is the only star where we have a chance to identify and study in detail the processes responsible for changes in irradiance on time scales from minutes to decades-the longest time scale over which high precision data are available. High-resolution spatial and temporal observations conducted in various space and ground-based experiments demonstrate that the surface of the Sun and its outer atmosphere are highly variable on almost all spatial scales, and that many of the observed changes are linked to interior processes taking place in the Sun’s convective zone or below. The broad collection of the material in this Monograph clearly shows that the variable solar energy output affects the Earth’s atmosphere and climate in many fundamental ways. However, a quantitative understanding of all the involved processes and their relationship to the climate system and its response remains elusive. Based on the current database and knowledge, it remains to be seen what role solar forcing will play in future climate.