This paper describes the mechanism of calcium and phosphorus homeostasis in dairy cows in an effort to provide a clearer understanding of the rationale behind current management and supplementation practices. Specifically addressed is the need to keep prepartum dietary calcium intake at less than or equal to 50 g/d to minimize the incidence of milk fever. Also discussed is the need to increase National Research Council recommendations for postpartum dietary calcium from 2.7 to 3.4 g/kg milk. This is particularly important during the first 1 to 2 mo of lactation to maintain calcium balance.