The success of mammographic screening for breast cancer is that it involves increasingly more patients with small primary tumors formerly thought to have an overall excellent prognosis. Yet, only approximately two thirds of these patients actually have this favorable prognosis, while the remaining third develops metastatic disease. Thus, there is emerging evidence that epithelial tumor cells can disseminate into secondary organs at an earlier stage of primary tumor development than appreciated by current risk classifications. Bone marrow is one of the most prominent secondary organs screened for the presence of disseminated tumor cells. The current data suggest that bone marrow micrometastases represent a selected population of dormant and heterogeneous cancer cells. The analysis of micrometastatic cells opens a new avenue by which to assess the molecular determinants of both early tumor cell dissemination and subsequent outgrowth into overt metastases. Moreover, identifying therapeutic target structures (e.g., HER2/neu), monitoring the elimination of bone marrow micrometastases, and assessing treatment-resistant tumor cell clones might help to understand the current limitations of adjuvant systemic therapy. This review summarizes the current knowledge of the biological characteristics of micrometastatic cancer cells in bone marrow of breast cancer patients.