A circulating tumor cell (CTC) is a cell that has shed into the vasculature or lymphatics from a primary tumor and is carried around the body in the blood circulation. CTCs can extravasate and become seeds for the subsequent growth of additional tumors (metastases) in distant organs, a mechanism that is responsible for the vast majority of cancer-related deaths. The detection and analysis of CTCs can assist early patient prognoses and determine appropriate tailored treatments. Currently, there is one FDA-approved method for CTC detection, CellSearch, which is used to diagnose breast, colorectal and prostate cancer.The detection of CTCs, or liquid biopsy, presents several advantages over traditional tissue biopsies. They are non-invasive, can be used repeatedly, and provide more useful information on metastatic risk, disease progression, and treatment effectiveness. For example, analysis of blood samples from cancer patients has found a propensity for increased CTC detection as the disease progresses. Blood tests are easy and safe to perform and multiple samples can be taken over time. By contrast, analysis of solid tumors necessitates invasive procedures that might limit patient compliance. The ability to monitor the disease progression over time could facilitate appropriate modification to a patient's therapy, potentially improving their prognosis and quality of life. The important aspect of the ability to prognose the future progression of the disease is elimination (at least temporarily) of the need for a surgery when the repeated CTC counts are low and not increasing; the obvious benefits of avoiding the surgery include avoiding the risk related to the innate tumor-genicity of cancer surgeries. To this end, technologies with the requisite sensitivity and reproducibility to detect CTCs in patients with metastatic disease have recently been developed. On the other hand, CTCs are very rare, often present as only a few cells per milliliter of blood, which makes their detection rather challenging. In addition, they often express a variety of markers which vary from patient to patient, which makes it difficult to develop techniques with high sensitivity and specificity.