SMARTS is a simple model of the atmospheric radiative transfer of sunshine. It can predict the direct beam, diffuse, and global irradiance incident on surfaces of any geometry at the Earth’s surface. It covers the whole shortwave solar spectrum (280 to 4000 nm), and thus includes the UVA, UVB, Visible and Near-Infrared bands. Besides the regular irradiance predictions needed for many possible applications, it can be used to simulate the spectral or broadband irradiance that would be measured in the solar spectrum.