The detection of disseminated tumor cells in bone marrow and blood is increasingly used for staging and therapeutic decisions in breast cancer and other solid tumors. Molecular biological methods improve the diagnostic accuracy. Limitations of the approach relate to the lack of disease-specific marker genes. The detection of tumor cells in the bone marrow after primary therapy is a negative prognostic parameter in many solid tumours. Axillary lymph node dissection and histopathology remain the standard staging procedure in breast cancer, but nodal negative patients exhibiting tumor cells in the bone marrow have an inferior outcome and may benefit from adjuvant therapy. The immunohistochemical and molecular detection of tumour cells in lymph nodes reduces the number of truly nodal-negative patients considerably. Tumour cells in bone marrow and blood may be used to directly monitor therapeutic responses.