Given the technological constraints of accurately measuring Ca and the negative consequences of hypocalcemia on cow health and farm economics, many dairy cows are supplemented with Ca immediately after calving. Administration of oral Ca, generally in bolus form containing 40 to 50 g of Ca, is a common supplementation strategy both for prevention and treatment of subclinical hypocalcemia, and several manufacturers have introduced oral Ca boluses containing differing combinations of rapidly and slowly absorbed Ca salts. The benefit of rapidly absorbed salts, such as Ca chloride, is that it is both highly bioavailable and acidifying (supporting mobilization of the cow’s own Ca stores); however, it is irritating to oral mucous membranes and must be administered quickly. Slowly absorbed Ca salts (such as Ca propionate, Ca sulfate, and Ca carbonate) have either an equivalent efficacy and longer duration of action than quickly-absorbed Ca salts or are ineffective as an immediate Ca source due to their poor bioavailability. Depending on the product, label instructions require administration of 1 to 3 boluses at calving, with some products requiring a second bolus administered 12 h later. Although studies agree that oral Ca increases blood Ca concentration, the length of increase varies from 1 to 24 h, likely due to the dose and frequency of administration and the production capacity of the cows under study. It is interesting that this short-term change in blood Ca concentration can have effects on cow health and production. However, these effects are not always beneficial, and most studies do not recommend their use as a blanket treatment, especially in primiparous cows. Conversely, there is good evidence that oral Ca supplementation to older cows and those with a greater production potential is valuable. It is thus important to take the formulation of oral Ca boluses, the timing and frequency with which they are administered, and cow-level variables into account when using oral Ca for treatment or prevention of hypocalcemia in dairy cows. It is important that these supplements complement the progression of, but do not interfere with, Ca homeostasis.