Abstract Decision-making for the medical oncologist was once far simpler than it is today. Following the 2000 NIH Consensus Conference on the treatment of early stage breast, almost all women with tumors > 1cm or positive lymph nodes were offered a course of adjuvant chemotherapy. There were some choices to be made in terms of the specific adjuvant regimen, but the oncologist could rely upon the results of large, randomized trials and choose a regimen that had been shown to be beneficial. Indeed, some might argue that a medical oncologist in 2009 could take the same approach. But the breast cancer world has evolved substantially over the past 5-7 years, and such an approach would not take full advantage of our understanding of breast cancer in 2009. We now understand that there is a great degree of intrinsic heterogeneity across breast tumors, and that the benefits associated with adjuvant chemotherapy vary substantially by subtype of disease. At minimum, we should consider three groups of patients separately: 1) individuals with HER2 positive disease; 2) individuals with triple negative disease; and 3) individuals with ER+ and HER2 non-amplified, non-overexpressed disease, who represent the vast majority of the patients with breast cancer in the United States. For patients with HER2+ disease, trastuzumab-based therapy has become the standard of care for all but women who have very small (T1a) tumors in the setting of negative lymph. Even in those women, there are physicians who would recommend a course of adjuvant trastuzumab-based chemotherapy. The major question is what chemotherapy to use in combination with trastuzumab, and this is a matter of considerable debate. In addition, uncertainty exists surrounding the optimal treatment for individuals who have indeterminate HER2 test results. For women with triple negative breast cancer, chemotherapy is also given to almost all patients apart from those with the very smallest tumors in the context of negative lymph nodes. For most patients, one of the standard anthracycline-taxane regimens remains the usual course of therapy. For patients with a low disease burden, first or second generation regimens such as AC, TC, or CMF are reasonable considerations. The most difficult questions arise concerning the use of chemotherapy in women with ER+ and HER2 normal disease. In this setting, the average benefit associated with chemotherapy is far less than in women with HER2+ or triple negative disease. Recent retrospective studies suggest that there are large groups of women who likely derive almost no or almost no benefit from chemotherapy. Increasingly, assays such as the Oncotype Dx Recurrence Score are used to aid clinical decisions, but even these complex genetic assays do not always provide a clear roadmap. Controversies and practical considerations surrounding the use of chemotherapy in women with ER+ and HER2 normal disease will be discussed in detail. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr CS1-2.