Building a ‘smart city’ sounds like a good idea, but what lies behind a city’s IQ? Smart cities involve innumerable sensors of all kinds, gathering data in real-time throughout the city. Together, these sensors harvest data about every aspect of urban life, including temperature, air quality, traffic flows, and infrastructure issues. Some of the most valuable smart city data is about how individuals live, move, and interact within the city. Data about individuals is captured by security and traffic cameras, by transit cards, by sensors measuring their consumption or use of utilities and resources, and increasingly by sensors carried by individuals themselves in the form of mobile phones and apps.