Immunocytochemical or molecular assays allow the detection of single disseminated tumor cells (DTCs) in the bone marrow (BM) or the peripheral blood in 10% to 60% of breast cancer patients without signs of metastasis. Results from recently reported studies suggest that circulating tumor cell (CTC) levels may serve as a prognostic marker and be used for early assessment of therapeutic response in patients with metastatic breast cancer. In early stage breast cancer, however, the impact of CTCs is less well established than that of DTCs in BM, where several clinical studies demonstrated that such cells are an independent prognostic factor at primary diagnosis. The characterization of DTCs/CTCs has already shed new light on the complex process underlying early tumor cell dissemination and metastatic progression in cancer patients. Characterization of DTCs should help to identify novel targets for biological therapies aimed to prevent metastatic relapse. In addition, understanding tumor 'dormancy' and identifying metastatic stem cells might result in the development of new therapeutic concepts.