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Reasons for feeding supplements to cattle consuming forage-based diets include conservation of forage, improvement of animal performance, increasing economic return, and(or) managing cattle behavior. This review focuses on identifying the limiting nutrients, effects of source of carbohydrates in energy supplements, feed additives, and feeding systems. The first step in designing a supplement is to determine nutrients that limit utilization of forage energy. Although this seems simplistic, grazing cattle select their diet, and nutrient composition of consumed forage often is unknown. Blood urea nitrogen and fecal analysis offer point-in-time indicators of protein-energy balance of the diet. Energy supplements (balanced with protein, vitamins, and minerals) are often needed to meet desired performance. Several experiments indicate that when nonstructural carbohydrates (NSC) in supplements are offered above .4% of BW, forage intake and digestibility are reduced. In this situation, choosing a highly digestible supplement low in NSC gives 15 to 30% better performance per unit of supplemental TDN. Several antibiotics increase gain and efficiency of growing cattle fed forage diets. Supplementing approved levels of ionophores or bambermycins has improved gains .07 to .11 kg/d, an increase similar to feeding an additional .45 kg of TDN/d. Other antibiotics and feed additives that control diseases, bloat, and parasites may increase net returns in selected situations. Supplementing cattle on low-quality forages with cottonseed meal (7 kg/wk or less) three times or once each week usually gives similar performance compared to daily feeding. Infrequent, compared with daily, supplementation with low-protein grain usually results in lower performance for cattle on low-quality forages, but supplements with moderate protein levels may be successfully fed infrequently. Formulating self-fed supplements that are consumed at desired amounts reduces feeding costs, but challenges for the nutritionist include variation in consumption across situations, variation in consumption amount and frequency within the herd, and availability of reasonable cost intake limiters that do not cause management or disease problems. Salt has been a reliable intake limiter.