The measurement of global sea-surface temperature (s.s.t.) from space, with high absolute accuracy, is one of the important requirements of the World Climate Research Programme (W.C.R.P.). This paper considers the definition of measurement aims based on considerations of specific types of scientific problem, and gives as examples discussion of two particular problems, first the possible influence of Pacific s.s.t. on the lower stratosphere, and second the role of s.s.t. in the cloud-climate feedback process. Following this, a brief review is presented on current status in satellite measurements of s.s.t. with both infrared and microwave techniques, and the paper concludes with a description of a future s.s.t.-measuring instrument, the Along-Track Scanning Radiometer (ATSR).