The present book describes various forms of solar energy conversion techniques in a unified way. The physical framework used to describe the various conversions is endoreversible thermodynamics, a recently developed subset of irreversible thermodynamics. It thus studies situations which are not in equilibrium and in which therefore entropy is continuously created. Nevertheless the mathematics is simple, because we consider only stationary situations.Most undergraduate textbooks on thermodynamics emphasize equilibrium thermodynamics and reversible processes. No entropy is created and conversion efficiencies are maximal: equal to the Carnot efficiency. For irreversible conversion processes, the reader learns only that entropy production is positive and that conversion efficiency is lower than the Carnot efficiency. But how great the entropy creation is, and how low the efficiency, is usually not expressed. Endoreversible thermodynamics gives us the opportunity to calculate explicit values for a broad class of processes. It is demonstrated in the text that solar energy conversion is a process particularly suited to being described in this way.