Breast cancer is the leading cause of cancer death for women worldwide. While breast cancer incidence is lower for many ethnic minority women than for white women, stage at diagnosis and survival are often worse. These disparities are most marked for African-American women, but are also present for Asians, Latinas, Native Americans and Hawaiians. The etiology of ethnic disparities in breast cancer is multifactorial, including differences in tumor characteristics, genetics, access to care and insurance, prevalence of risk factors, screening participation and processes of care, such as timeliness of diagnosis and quality of communication and treatment. This review will examine what is known regarding ethnic differences in all of these areas, what questions remain, and where researchers and policy makers should focus their future efforts.