Early studies could not prove any diagnostic or prognostic value of the presence of tumor cells in the circulation. Recent knowledge in the field of molecular and cellular pathology provided better understanding of mechanisms of metastasis formation therefore advanced detection of circulating cancer cells has been suggested as a supplementary method of staging metastatic cancer. Beside the widely used immunocytochemical methods the reverse transcriptase-polymerase chain reaction (RT-PCR) is now the most relevant technique in studying micrometastases of solid tumors. Magnetic activated cell sorting (MACS) is a recently developed method for the enrichment of different cells from suspensions by magnetic labelling of their surface antigens. RT-PCR seems to be the most sensitive to detect circulating cancer cells or micrometastases, but it is possible by MACS to purify cells for further immunological, biochemical or genetic analysis. The aim of this review is to give a brief summary of recently used methods and to discuss the clinical relevance of the attainable results.