Despite the advances in early detection and treatment of cancer, patients continue to die of the disease even when they seek care at an early stage. For patients with breast cancer, it is now possible to detect circulating tumor cells (CTCs) in the bloodstream and disseminated tumor cells (DTCs) in the bone marrow by using immunocytochemical and molecular methods. CTCs and DTCs have been found to share similar genotypic and phenotypic characteristics with so-called breast cancer stem cells, a finding that could potentially explain the eventual relapse of disease in a patient previously considered to have been cured by primary therapy. In some studies, the presence of CTCs or DTCs at the time of diagnosis of breast cancer is an independent adverse prognostic variable. However, before CTC/DTC testing can achieve standard-of-care status, there must be improvement in the sensitivity, precision, and reproducibility of the detection methods.