We live next to a star. The variability of that star affects our life hi different ways, as it induces climate changes or subjects spacecraft to the effects of mass ejections. The variability of the Sun provides a unique opportunity to observe processes that occur throughout the universe on scales that are too vast to imagine or to study in a laboratory on Earth, and that are too complicated to simulate in computer experiments in all their detail. Time and again, the Sun has served as the foundation for our thinking about stars and other astrophysical objects; many concepts (including magnetic activity, sfarspots, eoronae. flares, asteroseismology, ...) and processes (such as nuclear processes, radiative transport, stratified turbulent convection, dynamo, magnetoconveetive coupling, magnetic reconnection, ....) spring from solar physics. The Sun continues in that role as our ability to observe the Sun has advanced dramatically in recent, years, as have our resources for numerical approximations. Our new view of the Sun is one of an intrinsically dynamic, highly structured object, in which waves and radiative transfer, chemical inhomogeneity and multi-fluid effects are unavoidable realities.