Breast cancer is a heterogeneous disease with vast differences between patients regarding treatment response and prognosis. Therefore, strategies for individualizing care are needed. The rapid developments in biomarker research in breast cancer are making personalized breast cancer therapy a reality. A biomarker is defined as an objectively measured characteristic that can be evaluated as an indicator of normal biologic processes, pathogenic processes, or therapeutic responses. Biomarkers can have prognostic or predictive value. A small group of individual biomarkers has been used in the management of breast cancer, including estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2. Advances in molecular biology and an increased understanding of tumor cell biology have led to the discovery of a vast array of promising new biomarkers, including cancer stem cells, circulating tumor cells, gene-expression profiles, individual response markers, disease subtypes, predictors of metastasis, and mutation markers. To be adopted into routine practice, these candidate biomarkers will require extensive clinical validation. The improved application of traditional biomarkers and the discovery of additional markers will undoubtedly change the face of breast cancer care.