The lack of an adequate radiation measurement network in many parts of the world still necessitates the estimation of incoming radiation from the length of the burn in suncards from the Campbell-Stokes sunshine recorder. Currently used solar-radiation: sunshine formulations are therefore critically reviewed. The concepts of the so-called sunhours model of estimating incoming radiation, using only temperature, relative humidity and sunshine data, are explained before the inputs into the model are discussed. Particular attention is paid to estimating from a physical basis the atmospheric transmissivity as well as accounting for attenuation by cloud and for the diffuse radiation component. Tests of the model against radiation measurements yield highly satisfactory results for a variety of seasonal and radiation conditions, and show improvements over two frequently used radiation : sunshine-data relationships. Finally the use of the model for estimating the diurnal variation of incoming radiation intensities is illustrated.