Cancer cells spread through the circulatory system as “circulating tumor cells” (CTCs) and represent unique diagnostic targets. CTCs are released from primary tumors into blood circulation and then lodge at distant organs to initiate new cancer growth. Thus circulating tumor cells constitute the major cause for mortality among cancer patients. Recent advancements in detection and characterization of CTCs can have a significant impact on early cancer detection. For instance, the quantization of CTCs can be used for prognosis of the outcomes of metastatic diseases. In another example, identification of cancer stem cells, a subpopulation of CTCs believed to be tumorigenic, can be used for identifying metastasis and recurrence of disease.