The detection of disseminated tumor cells in bone marrow is a common phenomenon seen in 30-40% of primary breast cancer patients. The presence of disseminated tumor cells at diagnosis as well as the persistence of disseminated tumor cells is strongly associated with poor clinical outcome. Since bone marrow biopsies are not well tolerated by many patients, the evaluation of circulating tumor cells in the blood might become a desired alternative. Circulating tumor cells are routinely detected, depending on stage of the disease and methodology, in 10-80% of breast cancer patients. Recent studies have shown a prognostic potential of circulating tumor cells in both primary and metastatic settings. The evaluation of circulating tumor cells may become one of the crucial markers for prediction of survival and therapy monitoring, and its characterization might enable specific targeting of minimal residual, and metastatic disease.