Nearly 80% of New Yorkers and almost two-thirds of the U.S. population own a smart device with WiFi capability. The proliferation of these devices, and the increasing WiFi coverage offered by public networks, creates an opportunity to harness these data to better understand population dynamics and human mobility in cities. Given the increasing density and demand for urban space, new methods are needed to quantify real-time, localized populations and to understand their movement patterns. Such knowledge will inform studies of public health and exposure, urban transportation networks, emergency management and response, and the design of public space, among many other uses. This project uses large-scale locational data (over 1,000,000,000 records) to understand patterns of mobility across the U.S. We are working to understand evacuation and recovery during natural disasters, public space and park activity and utilization, neighborhood change and socioeconomic integration, and real-time population census to supplement existing survey-based methods.