Many modern buildings become uncomfortably warm during sunny spells in the summer, and until recently there was no simple, reliable method of assessing at the design stage whether a building would become overheated. This paper describes a method of calculating summertime temperatures which was developed at the Building Research Station, and gives data on parameters required for calculation. These include--(1) solar radiation intensities, (2) solar gain factors for glass, (3) blinds and other sun controls, and (4) 'admittances' of building components which are a measure of the ability of the components to smooth out diurnal temperatures in the building. (TC)