Prostate cancer is a progressive, multistep disease which presents many stages for intervention. Microscopic cancer is found in the prostate beginning by age 30 in about 20% of men, and the incidence increases steadily so that by the time a man is 90 years old, he has almost a 100% chance of having cancer in his prostate. Independent, multiple foci of cancer are present in the majority of prostate specimens, and the incidence of premalignant lesions is even higher than that of cancer. Yet, despite the high incidence of microscopic cancer, only 8% of men in the US present with clinically significant disease during their lifetime. Furthermore, only 3% of men in the US die of prostate cancer. In no other human cancer is there such disparity between the high incidence of microscopic malignancy and the relatively low death rate. Thus, there are many windows of opportunity for control of prostate cancer. Evidence from diverse areas of study – epidemiologic, molecular, genetic, cellular, animal models, and clinical trials – suggests that vitamin D may be an effective preventive agent against prostate cancer.