Abstract: Contact investigations have been a vital public health strategy, most recently in controlling tuberculosis and sexually transmitted infections including HIV. Yet, the sheer scale of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections poses major challenges to contact investigations. Strategies in China, Singapore, South Korea, and Taiwan have supplemented traditional manual approaches with digital surveillance through smartphone applications. The US has not used digital surveillance as a tool, but Google, Apple, the Massachusetts Institute of Technology (MIT), as well as 2 pan-European consortia and a variety of independent efforts are developing Bluetooth smartphone technology to enable rapid notification of users that they have had a close exposure to individuals diagnosed with medically verified coronavirus disease 2019 (COVID-19). How does digital tracking differ from manual tracing? Although digital surveillance has the distinct advantages of scale and speed, does it confer sufficient public health benefit to justify adoption given privacy concerns? How do the design choices of digital contact tracing systems affect public health and privacy?