Early spread of tumor cells is usually undetected by current imaging technologies. In patients with cancer and no signs of overt metastases sensitive methods have been developed to detect circulating tumor cells (CTCs) in the peripheral blood and disseminated tumor cells (DTCs) in the bone marrow. These technologies can be classified into cytometric and/or immunological and molecular approaches. Interestingly, the bone marrow seems to be a common homing tissue for cells derived from various epithelial tumors, and level 1a data from European and US groups have confirmed the prognostic impact of DTCs in the bone marrow of patients with breast cancer. Sequential peripheral blood analyses, however, are more convenient than bone marrow analyses for patients with solid tumors, and many research groups are currently assessing the clinical use of CTCs for assessment of prognosis and monitoring of systemic therapy. Molecular characterization of DTCs and CTCs opens a new avenue for understanding cancer dormancy, and might contribute to the identification of metastatic stem cells with important implications for future therapies. This Review focuses on the clinical relevance of the latest research results on blood-borne cancer micrometastases in patients with cancer.