Breast cancer risk is the chance of getting breast cancer. Differences in risk provide the foundation for the epidemiological study of breast cancer. Many aspects of women's diets and lifestyle have been studied for their effect on increasing or decreasing the risk of breast cancer. The aspects that are linked to breast cancer risk are known as risk factors. Risk factors are significant for two reasons. First, their understanding can provide important clues to the causes of a disease. Second, they can define public health measures that can lead to decreases in the incidence of a disease. This fact sheet examines the concept of risk, and how it is used in epidemiological studies to define risk factors. In addition, breast cancer risk factors are reviewed on the basis of the strength of the evidence supporting their association with breast cancer risk and the size of the associations.