Tumor biomarker tests are critical to implementation of personalized medicine for patients at risk for or affected by breast cancer. A tumor biomarker test must have high analytical validity and clinical utility to be used to guide clinical care in standard practice. Few tumor biomarkers meet these high standards. These include germline DNA single-nucleotide polymorphisms in the BRCA1 and -2 genes to determine high risk in unaffected women, selected tissue-based markers to determine prognosis and predict benefit from therapy, and circulating MUC1, CEA and perhaps tumor cells to monitor patients with metastatic disease. Efforts to discover biomarkers that predict therapeutic toxicity are promising but not yet successful. Further research is needed to enhance the number of tumor biomarker tests so that patients with breast cancer can get the correct treatment at the appropriate time.