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Circulating tumor cells (CTCs) are a subset of cancer cells that are shed from the primary or metastatic tumors into the bloodstream. CTCs are responsible for the establishment of blood-borne distant metastases but their rarity, estimated at one CTC per billion blood cells, presents the biggest technical barrier to their functional studies. Recent advances in CTC isolation technology have allowed for the reliable capture of CTCs from the whole blood of cancer patients. The ability to derive clinically relevant information from CTCs isolated through a blood draw allows for the monitoring of active disease, avoiding the invasiveness inherent to traditional biopsy techniques. This review will summarize recent developments in CTC isolation technology; the development of CTC-derived models; the unique molecular characteristics of CTCs at the transcriptomic, genomic, and proteomic levels; and how these characteristics have been correlated to prognosis and therapeutic efficacy. Finally, we will summarize the recent findings on several signaling pathways in CTCs and metastasis. The study of CTCs is central to understanding cancer biology and promises a “liquid biopsy” that can monitor disease status and guide therapeutic management in real time.