The chemokine CXCL12 (SDF-1) and its cognate receptor CXCR4 were first identified in the context of trafficking and homeostasis of immune cells, such as T lymphocytes. Subsequently, it has been determined that CXCR4 regulates several key processes in a wide variety of cancers. Functions of CXCL12 and CXCR4 in cancer first were described in metastatic breast cancer, and more recent studies also have identified roles for this signaling pathway in primary breast tumors. This review focuses on functions of CXCR4 and CXCL12 in primary and metastatic breast cancer, including molecular mechanisms of action and relationships of this pathway to other key regulators of breast cancer progression. We also describe pre-clinical studies indicating the potential to exploit CXCR4 as a new molecular target for diagnosis and treatment of breast cancer in patients.