Learning early in life may play an important role in the development of dietary habits and foraging skills of domestic ruminants. In this review we (1) explore the theoretical and empirical bases of early dietary learning, (2) examine constraints on such learning, and (3) discuss the implications of diet learning for livestock production and range management. Ruminant livestock select a diet from a wide variety of potential foods; some are appropriate while others are nutritionally poor or even harmful. Because of these complexities, evolutionary processes should favor animals whose diet selection is based on generality and flexibility. Learning which foods to eat and which to avoid is such a system. One way for individual animals to discover the value of food is through trial and error learning. Their preferences for, or aversions to, foods may result from trial and error learning based on cautious sampling and resultant nutritional and physiological consequences. Additionally, evolutionary processes could provide selective pressure for herbivores that feed in large, mixed-generation groups to rely on social learning, where foraging information is passed from experienced to inexperienced foragers. Recent research suggests that the mother may greatly influence her offsprings' dietary habits. As a result, dietary learning may be more pronounced in early, as opposed to later, life and there may be a sensitive period that coincides with the transition from monogastric to ruminant; i.e. weaning. There are genetic, metabolic and morphological constraints on dietary learning. Plant olfactory and gustatory characteristics that ruminants have consistently associated with negative consequences may be genetically fixed, and it is often possible to explain avoidance of plant species and parts by herbivores in terms of deterrent phytochemicals and physical plant characteristics. It is considerably more difficult to identify feeding attractants in plants. In addition, small ruminants are constrained nutritionally to select diets of higher quality than large ruminants, and the efficiency with which small versus large ruminants ingest different plant parts and life forms may be different. Teh likelihood that domestic ruminants learn to forage presents both problems and opportunities for livestock and rangeland managers. On the one hand, learning may result in inefficient foragers when animals are moved to new environments. On the other, through the manipulation of past dietary experience, a manager may create a foraging group that better fits specific management goals.