One of the primary challenges in the treatment of patients with early-stage breast cancer is determining which patients will benefit from adjuvant chemotherapy. Traditionally, treatment decisions have been made based on a combination of tumor characteristics and patient and physician perspectives regarding risks and benefits. Recent technologic advances, including the development of gene-expression arrays, have led to the identification of molecular signatures that provide prognostic information in addition to the basic clinicopathologic features of individual tumors. While these new methods allow for more refined determination of prognosis for an individual patient, few data are available to support use of these new technologies in the clinic for treatment decision-making. At present, data from a single retrospective study are available to support the use of one assay, the 21-gene recurrence score, for decision-making regarding adjuvant chemotherapy. Large, multinational clinical trials are currently ongoing to evaluate the use of two of the multiparameter assays, although it will be many years before oncologists can apply the results of these trials in the clinic.