Circulating tumor DNA (ctDNA) is a component of the “naked” DNA found in blood. It can be isolated from plasma and represents combined genetic material from the primary tumor and metastases. Quantitative and qualitative information about a cancer, including mutations, can be derived using digital polymerase chain reaction and other technologies. This “liquid biopsy” is quicker and more easily repeated than tissue biopsy, yields real-time information about the cancer, and may suggest therapeutic options. All stages of cancer therapy have the ability to benefit from ctDNA, starting with screening for cancer before it is clinically apparent. During treatment of metastatic disease, it is useful to predict response and monitor disease progression. Currently, ctDNA is used in the clinic to select patients who may benefit from epidermal growth factor receptor–targeted therapy in non–small cell lung cancer. In the future, ctDNA technology promises useful applications in every part of clinical oncology care.