Carcinomas of epithelial origin represent the majority of malignancies in Europe. A substantial number of patients develop recurrent carcinoma, which is explained by tumor-cell dissemination into distant organs, preferentially bone marrow, which often occurs prior to surgery. In contrast to disseminated tumor cells in bone marrow, for which the prognostic value has been demonstrated, the role of circulating tumor cells (CTCs) in blood is not yet completely understood. Since bone marrow aspiration is less accepted by patients than blood withdrawal, it would be highly desirable to replace bone marrow aspiration by blood analysis. Presently, a variety of seemingly promising methods for the detection and characterization of CTCs are under evaluation, including immunocytologic and molecular approaches. However, these methods still need to be proven useful in clinical studies. The majority of studies published on CTCs to date have been related to primary and metastatic breast cancer; therefore, this article mainly addresses the role of CTCs in breast cancer.