A general problem of animal production is that of predicting the growth rate, body composition, and feed intake of any kind of animal, in any state treated in any way. The solution to this problem depends, in part, on devising a sufficient description of the animal. Firstly, we considered the ability of the Gompertz form to describe the potential growth of an animal when conditions are not limiting. Experimental results from 2 lines of Suffolk sheep, one selected for lean growth rate and the other its control, were used to quantify and test the function. The Gompertz function fitted the data well, with no pattern in the residuals; thus, it is useful for describing potential growth. Secondly, in practice, growth may be limited by level of feeding or dietary protein, or many other things. Thus, we considered several approaches to describing growth when conditions were limiting. The functional form relating weight to cumulative food intake proposed by Spillman did not provide a satisfactory description of the data either on controlled or ad libitum feeding. In the Suffolk experiment, we found a higher sensitivity of the selected line in growth rate and feed efficiency to reductions in both the amount given and food protein content. A reduction in level of feeding reduced fatness, whereas a reduction in the protein content of the food increased it; effects on lean content were in the opposite direction. Importantly, the advantages in lean and fat contents in the carcass strongly persisted in the selected line regardless of the nutritional environment. This suggests that genetic selection to improve carcass lean growth rate will yield leaner carcasses across a wide range of finishing systems, even though these finishing systems themselves may affect the fatness of the carcass. Therefore, consumers are always likely to see benefits of selection in terms of reduced carcass fatness. However, since the benefit of improved genotypes may not be expressed in terms of live performance, grading and pricing systems are needed which reward producers for using superior sires.