Descriptive epidemiologic data suggest a relationship between consumption of high fat diets and breast cancer; although these data can be potentially confounded by other causative exposures. Results of published case-control and cohort studies are inconclusive. Nevertheless, dietary fat significantly affects mammary tumorigenesis in mice and rats in laboratory experiments. We will review current epidemiologic and animal studies, explain the possible mechanisms of how dietary fat may affect breast cancer, and provide preliminary dietary recommendations.