For this article, the New York Times bought millions of data points from a data broker. In New York City alone, the data for a period of three days show 235 million locations recorded by 1.2 million devices. Dozens of companies use our cellphones’ location services to improve their advertising. These data are allegedly anonymized, but an analysis shows how easily they can be connected to individual people. Using the example of a middle school teacher, the New York Times discovered all the things that could be learned from her data, including her work schedule, visits to doctors or Weight Watchers, and even walks with her dog.