Circulating tumor DNA (ctDNA) has garnered much excitement over the past few years for its potential clinical utility as a surrogate for tumor biopsies in early cancer detection and prognosis. Numerous studies have demonstrated that ctDNA is shed into the circulation and is elevated in disease states such as cancer. Despite the low levels of ctDNA in the “sea” of normal DNA, advances in next generation sequencing (NGS) and digital polymerase chain reaction (PCR) technologies have led to dramatic improvements in variant detection sensitivity and specificity. These technologies allow the quantification of ctDNA, providing both prognostic and predictive information. Here, we review the history of cell-free DNA and different technologies for the detection of ctDNA in cancer and describe the different modalities for using ctDNA in clinical oncology.