Nutrient requirements are used by many groups to determine food choices and meal plans for feeding people across the lifespan. They are used to evaluate sources of nutrients and for food labels to educate the consumer. Requirements also provide a guideline to determine whether supplements are needed. If calcium requirements are underestimated, inadequate intakes are more likely, which may compromise bone health and increase risk of chronic disease. The evidence supporting calcium requirements is stronger than for most nutrients. Yet, we lack consensus on the appropriate approach for determining calcium requirements. The various approaches for determining calcium requirements are reviewed in this chapter. We conclude that efficacy studies including well-controlled feeding studies are best for determining calcium intake requirements, and effective research including randomized controlled trials is best for evaluating how well various interventions work in translating the results of efficacy studies to the individual or community.