Breast cancer is the most common cancer affecting women, with approximately 200,000 newly identified cases every year in the United States. If identified while localized, treatment of breast cancer has a 98.6% 5-year survival rate; however, this drops to 83.8% once the cancer has spread to the lymph nodes (node positive, or N+), and drops further to 23.4% if the cancer has metastasized. Adjuvant chemotherapy is used in the treatment of many patients with a high risk of breast cancer recurrence to reduce the risk of recurrence following surgical resection. Physicians and patients must balance the benefits of adjuvant chemotherapy and antiestrogen therapy in terms of reducing the risk of cancer recurrence with the risk of toxicities associated with the use of these agents. Common toxicities of breast cancer chemotherapies are nausea and vomiting, myelosuppression, hair loss, mucositis, and cognitive decline. Less common adverse effects include heart failure, thromboembolic events, premature menopause, and leukemia. Given the need to balance the benefits and harms of adjuvant chemotherapy, it is generally offered on the basis of risk factors that have been shown to be associated with greater disease-specific mortality or greater chance of distant recurrence (metastasis), such as patient age, menopause status, disease stage and location, histologic and nuclear tumor grade, hormonal receptor status (ER, PR, and HER2), and measures of proliferation of the tumor. Several guidelines and prognostic models have been developed to aid physicians and patients in making these complex decisions. Three of the most common guidelines are the National Comprehensive Cancer Network (NCCN) guidelines, St. Gallen guidelines, and the National Institutes of Health (NIH) Consensus Conference Statement(2000) guidelines. Recently, several molecular assays have also been developed to provide prognostic information on which patients may benefit from adjuvant chemotherapy. The MammaPrint assay is one such test.