A conceptual framework is introduced which has been derived from work on insects. The scheme is intended to integrate studies of diet selection, regulation of amounts eaten, nutrient utilization, body composition and animal performance. Aspects of the framework are illustrated with published data on macronutrient selection in the rat. An animal is viewed as moving through a multidimensional nutrient space, which is bounded by axes representing each required nutrient and within which lie optimal points of intake and nutrient allocation (“targets”). The aim is first to estimate the location of these functional optima experimentally, and then to interpret the responses of animals which are constrained from reaching them (“decisions of best compromise”). The framework can then be used to interpret data from animals reared under differing environmental conditions and to compare animals of differing developmental stage, genotype or nutritional state.