Racial/ethnic disparities in breast cancer outcomes have been well documented; however, the reasons why certain groups have widely different health experiences are not well understood. Recognizing that variation in quality of cancer care may correlate with socio-demographic and health system characteristics, the overall objectives of this dissertation were (1) to investigate the relationships between race/ethnicity and structural/organizational aspects of health services in terms of post-operative receipt and timing of initiation of radiation therapy and adjuvant chemotherapy, and (2) to determine whether timing of adjuvant therapy initiation affects mortality. This dissertation used population-based SEER-Medicare data to examine these issues in female Medicare beneficiaries ages 65 and older diagnosed with primary breast cancer in the years 1994 to 2002. Structural/organizational variables examined included characteristics of the surgical facility (i.e., type/ownership, teaching status, size, institutional affiliations, and presence of on-site radiation services), distance traveled to surgical facilities, distance to nearest radiation therapy provider, and distance to nearest chemotherapy provider. Racial/ethnic groups examined included non-Hispanic white, non-Hispanic black, and Hispanic patients. We found significant racial/ethnic disparities in terms of receipt and timing of initiation of radiation therapy, as well as all-cause and breast cancer specific mortality, whereas we found no evidence of racial/ethnic disparities in adjuvant chemotherapy. We also found evidence that certain health services characteristics, including type/ownership and size of surgical facility, presence of on-site radiation at surgical facility, and distance from patient residence to adjuvant therapy providers, were associated with quality of care received, suggesting that health care systems or policies may be designed in such a way to improve outcomes for all breast cancer patients, and particularly, among minority women at risk for undertreatment. Finally, we found evidence that earlier initiation of radiation therapy and adjuvant chemotherapy may correspond to better health outcomes. This study documents the important role that health services characteristics may play in determining quality of care. Additionally, considering that black women are more likely to be diagnosed with aggressive, advanced stage cancers and more likely to die from breast cancer, this study suggests that earlier initiation of treatment may help minimize racial disparities in breast cancer mortality.