The failure to reduce mortality of epithelial cancer patients is probably a result of the early dissemination of cancer cells to secondary sites, which is usually missed by conventional diagnostic procedures used for tumor staging. Individual carcinoma cells present in regional lymph nodes, blood, or distant organs (eg, bone marrow) can be detected by sensitive immunologic or molecular methods. Because the goal of adjuvant therapy is the eradication of occult micrometastatic tumor cells before metastatic disease becomes clinically evident, the early detection of micrometastases could identify those patients who might benefit from adjuvant therapy. In addition, more sensitive methods for detecting such cells should increase knowledge about the biologic mechanisms of metastasis, which might improve the diagnosis and treatment of micrometastatic disease. In this article, the recent developments in sensitive assays used for the detection of individual micrometastatic cancer cells in patients with epithelial tumors are reviewed.