Breast cancer is the most common cancer in women, accounting for 32% of all newly diagnosed cancers. Demographic characteristics associated with an increased risk include increasing age, birth in North America or northern Europe, high socioeconomic status, never having been married, and, for breast cancer diagnosed after 45 years of age, the white race. Early age at menarche, late age at menopause, late age at first full-term pregnancy, and low parity increase risk, while removal of the ovaries at an early age is protective. Obesity increases risk in postmenopausal women. Having a first degree relative with breast cancer confers an increased risk, especially if both a mother and a sister have had breast cancer at an early age. Mutations in the BRCA1 and BRCA2 genes are associated with an inherited susceptibility to breast cancer at an early age. Other markers of increased risk include atypical and hyperplastic epithelial cells in nipple aspirate fluid, nodular densities on mammogram, and biopsy-confirmed benign proliferative breast disease. Little can be done at present to reduce breast cancer risk through primary prevention, but secondary prevention by mammographic screening in women of age 50 and older reduces mortality from breast cancer.