Circulating tumour cells (CTC) and circulating endothelial cells (CEC) are considered promising biomarkers for tumour aggressiveness and vascular damage. Their number in the blood of cancer patients has already demonstrated to be of prognostic and predictive value in several studies. In addition, the molecular characterization of these cells may provide novel insight in the growth and dissemination of cancer, and help the development of new anti-neoplastic drugs. Therefore, many attempts have been made to reliably detect these extremely rare cells in the blood. In this review, we address the detection, applicability and results from clinical trials for both CTC and CEC.