This review presents and discusses the nutritional and physiological aspects of cattle production under grazing in the tropics. The critical evaluations were based on Brazilian experimental results as well as on basic literature concerning ruminant nutrition. Several associations between the characteristics of the grazed forage, the composition of the supplements and animal and microbial requirements were established. The adopted approach was divided according to two different climatic seasons observed in the tropics: dry and rainy seasons. During the dry season, the main nutritional constraints on animal performance are associated with inherent requirements of the rumen fibrolytic microorganisms. An overall deficiency of nitrogenous compounds is observed in the forage, which compromises forage intake and digestibility. Supplementation with nitrogenous compounds must be a priority in this season to increase forage intake and energy extraction from forage fiber. However, during the rainy season, no constraints on microbial growth are observed. The usual pasture composition presents an unbalanced and high ratio of energy to protein when compared to animal requirements. In such cases, protein supplementation is needed to equilibrate the basal diet and improve the utilization of metabolizable energy and protein.