There is emerging evidence that epithelial tumor cells are able to disseminate to secondary organs at an early stage of primary tumor development. One of the most prominent secondary organs screened for this type of dissemination is bone marrow. Even in cancer entities where overt skeletal metastases are rare (e.g., colorectal and ovarian cancer), bone marrow is a prognostically relevant indicator organ for the presence of hematogenous micrometastases. The currently available data suggest that bone marrow micrometastases represent a selected population of dormant cancer cells which still express a considerable degree of heterogeneity. The analysis of micrometastatic cells will open a new avenue to assess the molecular determinants of early tumor cell dissemination and subsequent outgrowth into overt metastases. Moreover, monitoring the elimination of bone marrow micrometastases and identification of treatment-resistant tumor cell clones may help to increase the efficacy of adjuvant therapy. This review summarizes the current knowledge on the biological characteristics of micrometastatic cancer cells in bone marrow of patients with solid epithelial malignancies.