Current models suggest that at a certain but yet undefined time point of tumor development malignant cells with an aggressive phenotype start to disseminate via the blood stream into distant organs. This invasive phenotype appears to be associated with an epithelial-mesenchymal transition (EMT), which enables detachment of tumor cells from a primary site and migration and appears to be asscociated with stemness of cancer cells. The reverse process of mesenchymal-epithelial transition (MET) might play a crucial role in the further steps of metastasis when circulating tumor cells (CTCs) settle down in distant organs as disseminated tumor cells (DTCs) and establish (micro-)metastasis. Nevertheless, the exact mechanisms and interplay of EMT and MET are only partially understood and their relevance in cancer patients is unclear. Research groups have just started to apply EMT-related markers in their studies on CTCs in cancer patients. This lecture will summarize and discuss the current state of investigations on CTCs/DTCs in the context of research on EMT/MET and potential implications for the management of cancer patients.