Measurement science has been converging to smaller and smaller samples, such that it is now possible to detect single molecules. This Review focuses on the next generation of analytical tools that combine single‐molecule detection with the ability to measure many single molecules simultaneously and/or process larger and more complex samples. Such single‐molecule sensors constitute a new type of quantitative analytical tool, as they perform analysis by molecular counting and thus potentially capture the heterogeneity of the sample. This Review outlines the advantages and potential of these new, quantitative single‐molecule sensors, the measurement challenges in making single‐molecule devices suitable for analysis, the inspiration biology provides for overcoming these challenges, and some of the solutions currently being explored. More than the sum of its parts: Advances in measurement science have seen a progressive reduction in sample size to the point that single‐molecule measurements are today commonplace. A new generation of sensors is expected that perform quantitative analysis by measuring many single‐molecule events. This Review discusses the challenges, opportunities, and recent developments in quantitative single‐molecule sensors.