The similarities between the mouse and farm livestock at the genetic and functional level make it a useful model for farm livestock breeding and indeed for human genetic research, taking the opportunities presented by its short generation interval, the development of inbred lines, and the ability to do gene knock-outs. Genetic similarities apply both at the molecular level, in gene structure and sequence, and at the quantitative level, for example genetic parameters of growth. We discuss here what can be learnt about the action and interaction of genes that influence traits of growth and body composition and contribute to genetic changes in them from work on the mouse, concentrating particularly on candidate genes and experiments conducted in our laboratory. As a resource we have developed highly divergent selection lines for growth, body composition and food intake, brought together lines of mice selected for growth from around the world, and inbred them both for stability and to facilitate their use for molecular genetic analysis. These lines differ greatly as a consequence of selection not only in growth and degree of fatness, but also in efficiency, as assessed by food intake corrected for body weight.