A meeting entitled “Circulating Tumor Cells (CTCs): Emerging Technologies for Detection, Diagnosis, Prognosis and Treatment” was held in Bethesda in September, 2009 and was sponsored jointly by CDP, the NCI Division of Cancer Prevention and the NCI Small Business Innovation Research Program. The meeting brought together more than 500 participants: cancer biologists, clinicians, and the developers of technologies for detecting and collecting circulating tumor cells. Although it has long been known that cells shed from tumors can be found in the bloodstream, only recently has it been clear that measuring the burden of CTCs with automated counting instruments provides prognostic information that is useful clinically. Emerging technologies promise to increase the sensitivity of CTC detection and to permit pathologists to capture the cells and characterize them in many ways. In theory, blood tests based on CTCs could enable doctors to identify biological changes in a patient’s cancer as it progresses and to closely monitor its response to treatments. The focus of the workshop was on establishing more productive collaborations between the developers of new technologies, who come from both academic laboratories and from small companies, and the clinical researchers conducting trials that integrate measurement of CTCs into clinical decision making. Researchers at the meeting were joined by staff from FDA who made presentations on the path to regulatory approval of new medical devices.