Tumor cell dissemination appears even in patients with small solid tumors, and bone marrow (BM) is a common homing organ for disseminated tumor cells (DTC) derived from various types of primary epithelial tumors. Tumor cells are frequently detected in the BM of cancer patients without any clinical or even histopathological signs of overt metastases. It is crucial, however, to improve and standardize methods for the detection of DTC. The characterization of DTC has shed new light on the process underlying early tumor cell dissemination and metastatic progression in cancer patients. Characterization of DTC should help to identify novel targets for biological therapies aimed at preventing metastatic relapse and to monitor the efficacy of these therapies. Evidence has emerged that the detection of DTC and circulating tumor cells (CTC) in blood may provide important prognostic information and, in addition, might help to monitor the efficacy of therapy. In this article, we summarize the clinical background and the technical aspects of current methods used for the detection and characterization of DTC in BM and CTC in blood, with a special focus on breast cancer.