Circulating tumor DNA (ctDNA) has been making headlines for its potential applications in oncology, including multicancer early detection tests for cancer screening, minimal residual disease (MRD) testing for localized cancers, and treatment selection and response monitoring for advanced cancers. These developments can be understood as diverse applications of a tool that fundamentally expands the spectrum of malignant disease that can be measured and monitored. A prominent example of a similar tool in clinical practice is prostate-specific antigen (PSA), which has fundamentally shaped the management of prostate cancer. The advent of ctDNA has far-reaching implications for the future of solid tumor oncology because, to paraphrase a well-known business aphorism, you can only manage what you can measure.