Recent epidemiologic research suggests that dairy product intake or its components (calcium, vitamin D, and amount or source of protein) are associated with lower body weight or body fat. Clinical intervention trials designed to test this association during weight loss are promising, but still controversial. Few data are available on the effect of calcium or dairy products on prevention of weight gain in long-term trials. The mechanisms proposed to mediate the putative effect of dietary calcium are primarily the formation of fecal fatty acid complexes to reduce fat absorption and the regulation of energy metabolism, including lipolysis from adipocytes and fatty acid oxidation, through the calciotropic hormones, parathyroid hormone, and 1,25-dihydroxyvitamin D. Increased energy expenditure, increased satiety, or a shift from fat to lean mass must accompany these changes in lipid metabolism to achieve changes in fat mass; however, measurable changes in these other parameters either have not been tested or have not been noted uniformly. If dairy products or their components have an effect on altering fat mass, it is likely to be a small change that may have a substantial effect on the incidence of obesity over time.