Breast cancer is a cancer that starts in the cells of the breast. Worldwide, breast cancer is the second most common type of cancer after lung cancer (10.4% of all cancer incidence, both sexes counted) and the fifth most common cause of cancer death. However, among women worldwide, breast cancer is by far the most common cause of cancer, both in incidence and death. In 2005, breast cancer caused 502,000 deaths worldwide (7% of cancer deaths; almost 1% of all deaths). The number of cases worldwide has significantly increased since the 1970s, a phenomenon partly blamed on modern lifestyles in the Western world.

Breast cancer incidence is much higher in the Western world, whether in Europe or North America, than in third world countries. North American women have the highest incidence of breast cancer in the world. Among women in the U.S., breast cancer is the most common cancer and the second-most common cause of cancer death (after lung cancer). Women in the U.S. have a 1 in 8 (12.5%) lifetime chance of developing invasive breast cancer and a 1 in 35 (3%) chance of breast cancer causing their death. In 2007, breast cancer was expected to cause 40,910 deaths in the U.S. (7% of cancer deaths; almost 2% of all deaths).

In the U.S., both incidence and death rates for breast cancer have been declining in the last few years. Nevertheless, a U.S. study conducted in 2005 by the Society for Women's Health Research indicated that breast cancer remains the most feared disease, even though heart disease is a much more common cause of death among women.

Because the breast is composed of identical tissues in males and females, breast cancer also occurs in males. Incidences of breast cancer in men are approximately 100 times less common than in women, but men with breast cancer are considered to have the same statistical survival rates as women.