The rapidly growing array of therapeutic options in cancer requires informative biomarkers to guide the rational selection and precision application of appropriate therapies. Circulating biomarkers such as circulating tumor cells have immense potential as noninvasive, serial 'liquid biopsies' that may be more representative of the complete spectrum of a patient's individual malignancy than spatially and temporally restricted tumor biopsies. In this review, we discuss the current state-of-the-art in the isolation and molecular characterization of circulating tumor cells as well as their utility in a wide range of clinical applications such as prognostics, treatment monitoring and identification of novel therapeutic targets and resistance mechanisms to enable real-time adjustments in the clinical management of cancer. The evolving landscape of cancer therapy creates an urgent need for minimally invasive biomarkers to facilitate early detection, prognosis and prediction of therapeutic response and resistance to enable highly adaptable, real-time precision therapy [1]. The conventional gold standard of using single biopsies of the primary tumor to inform all downstream therapeutic decisions may no longer be adequate given increasing evidence of significant spatial and temporal hetero geneity in tumors [2], especially when placed under the selection pressure of various treatments. Moreover, serial biopsies are often impractical, morbid and technically challenging.In this review, we focus on the emerging role of circulating tumor cells (CTCs), which can be serially obtained from minimally invasive blood draws or 'liquid biopsies'. CTCs are cancer cells that have been shed into the peripheral circulation from primary or metastatic tumors [3,4]. The first reported observation of CTCs was in 1869 by Australian pathologist Thomas Ashworth at the autopsy of a patient with metastatic cancer [5]. By comparing the morphology of circulating cells with those from different cancerous lesions, Ashworth came to the prescient conclusion that 'cells identical with those of the cancer itself being seen in the blood may tend to throw some light upon the mode of origin of multiple tumors existing in the same person' [5]. Indeed, molecular analyses of CTCs may be superior to individual tumor biopsies because these circulating cells likely provide a sampling of different regions of the primary tumor as well as metastatic deposits, and therefore are more likely to capture the genetic heterogeneity of a patient's cancer.While cell-free circulating tumor DNA (ctDNA) is an important, complementary biomarker to CTCs, it has been reviewed extensively elsewhere and will not be discussed in this review [4,6]. Moreover, although ctDNA may in some cases be more reliably [7], they are largely limited to genomic mutational analysis. In contrast, CTC analysis can provide a wealth of other information including cell morphology, immunocytochemical phenotype, presence of important epitopes, presence of multiple mutations within a single cell to decode ...