This chapter reviews studies on the link between dietary fat and breast cancer. These include ecologic studies, migrant studies, special populations, secular trends, case-control studies, and prospective studies. The integration of findings from case-control and prospective studies, the re-examination of animal data, and specific types of fat and breast cancer are discussed. There are good reasons to reduce the intake of animal and partially hydrogenated fat. However, existing data provide little evidence for the hypothesis that reduction in dietary fat composition even to 20% of energy during adulthood will led to a significant decline in breast cancer.