Today's smartphones know almost everything about us: they capture who we know, who we communicate with, what apps we run, and where we go.

On the one hand, this rich information offers us the opportunity to understand people's behaviors and social interactions at a fidelity and scale that simply was not possible before. This kind of information can be very useful for social psychologists, urban planners, civil engineers, and city governments. In this talk, I will discuss some of the work we have been doing in using location data to infer friendships between people, showing how location entropy can be used to characterize the social quality of a place. I will also discuss Livehoods, our first urban analytics tool designed to help people understand the character, structure, and dynamics of cities.

On the other hand, this same information poses serious privacy threats. Many smartphone apps can be considered spyware, in terms of the amount and range of information they collect about us. I will also discuss some of our work in using crowdsourcing to dissect the behaviors of apps and to build better summaries that emphasize unexpected behaviors.