Gallium Arsenide (GaAs) has been of interest as a photovoltaic material for many years. This interest arises primarily for three reasons. First, the bandgap of 1.42eV at 300 K is very nearly ideal for a photovoltaic device operating in our solar spectrum. Second, GaAs solar cells should be capable of operating at higher temperatures than silicon (Si) cells. Third, GaAs solar cells are expected to be very radiation resistant. This handbook provides a history of GaAs solar cell development; Photovoltaic equations are described along with instrumentation techniques for measuring solar cells; Radiation effects in solar cells, electrical performance, and spacecraft flight data for solar cells are discussed; and the space radiation environment and solar array degradation calculations are addressed.