On January 23, 2012, the Supreme Court issued a landmark non-decision in United States v. Jones. In that case, officers used a GPS-enabled device to track a suspect’s public movements for four weeks, amassing a considerable amount of data in the process. Although ultimately resolved on narrow grounds, five Justices joined concurring opinions in Jones expressing sympathy for some version of the “mosaic theory” of Fourth Amendment privacy. This theory holds that we maintain reasonable expectations of privacy in certain quantities of information even if we do not have such expectations in the constituent parts. This Article examines and explores the mosaic theory and concludes that it exposes an important quantitative dimension of Fourth Amendment privacy but raises serious practical challenges, which, as we argue elsewhere, can be met by regulating surveillance technologies capable of facilitating broad programs of indiscriminate surveillance.