Incidence of solar radiation on an arbitrary surface is required for a variety of applications such as agricultural meteorology, photobiology, animal husbandry, daylighting, comfort air conditioning, building architecture and solar energy utilization. Estimation of insolation on a horizontal surface is the first step in almost all instances. Global radiation is recorded at more locations than its diffuse counterpart, and daily sunshine duration measuring stations are far more numerous than the radition measuring stations. The use is made of monthly averaged daily, individual daily, monthly averaged hourly and individual hourly irradiation according to the degree of sophistication required and the associated costs involved concerning man-hours and computing time. This paper reviews the progress made in this respect with particular reference to the United Kingdom. Theoretical proof of generally accepted concept, that the correlations between averaged and individual values of global and diffuse irradiation are fundamentally different, is also presented.