Abstract Multicancer early detection (MCED) tests may soon be available to screen for many cancers using a single blood test, yet little is known about these tests beyond their diagnostic performance. Taking lessons from the history of cancer early detection, we highlight 3 factors that influence how performance of early detection tests translates into benefit and benefit-harm trade-offs: the ability to readily confirm a cancer signal, the population testing strategy, and the natural histories of the targeted cancers. We explain why critical gaps in our current knowledge about each factor prevent reliably projecting the expected clinical impact of MCED testing at this point in time. Our goal is to communicate how much uncertainty there is about the possible effects of MCED tests on population health so that patients, providers, regulatory agencies, and the public are well informed about what is reasonable to expect from this potentially important technological advance. We also urge the community to invest in a coordinated effort to collect data on MCED test dissemination and outcomes so that these can be tracked and studied while the tests are rigorously evaluated for benefit, harm, and cost.