This paper reviews the differences between thermal and daylighting considerations for building design and their implications for using illuminance and irradiance data. Old and new mathematical models for luminance distribution and illuminance calculation are explained. Overcast, clear, and average sky conditions are discussed and their dependence on atmospheric factors is examined. We focus in particular on the turbidity of the clear atmosphere and the luminance at the zenith. We conclude that more measured daylighting data are required for validating the equations that have been developed separately in various parts of the world.