Every person is unique and complex. But can we predict anything about how large groups of people will behave? In the 1950s the writer Isaac Asimov imagined a field called “Psychohistory” that could mathematically describe the actions of large groups. I will argue that in a certain way, aspects of that vision are in wide use today. To illustrate, I will describe examples from my own work in which the “low dimensionality” of aggregate human behavior gives us leverage on a range of problems, from predicting network traffic to detecting anomalous or malicious behavior in social networks.