It is likely that returns from lamb production in future will depend, much more than at present, on producing leaner carcasses. There are several short-term changes in management which could produce leaner carcasses. However, In the longer term genetic Improvement, particularly by within-breed selection In terminal sire breeds, is likely to provide permanent, cumulative and cost-effective benefits In carcass composition. In the early 1980s a research project was started at the Edinburgh School of Agriculture, using Suffolk sheep, to examine the genetic potential for Improving carcass composition In terminal sires. The work commenced with an evaluation of techniques for in vivo measurement of carcass composition (Simm, 1987) and derivation of selection indices to incorporate In vivo measurements (Simm and Dingwall, 1989). Since 1985 In vivo measurement and Index selection have been practised In the experimental flock, which now numbers about 220 ewes. This paper reports the interim results of selection.