As metastatic "seeds," CTCs are believed to be responsible for dissemination from a primary tumor to anatomically distant organs. For the most epithelial tumors the presence of CTCs is associated with tumor progression, increased risk of local and regional recurrence as well as the development of distant metastases. CTCs could be used as a strong prognostic and predictive marker for patient's clinical outcome and survival. A considerable number of promising CTC-detection techniques based on molecular and immunologic methods have been developed in recent years. Most of them need an enrichment step to improve sensibility and/or specificity.