Smartphone apps have changed the way we interact with online services, but highly specialized apps come at a cost to privacy. In this paper we will demonstrate that a passive eavesdropper is capable of identifying finegrained user activities within the wireless network traffic generated by apps. Despite the widespread use of fully encrypted communication, our technique, called NetScope, is based on the intuition that the highly specific implementation of each app leaves a fingerprint on its traffic behavior (e.g., transfer rates, packet exchanges, and data movement). By learning the subtle traffic behavioral differences between activities (e.g., “browsing” versus “chatting” in a dating app), NetScope is able to perform robust inference of users’ activities, for both Android and iOS devices, based solely on inspecting IP headers. Our evaluation with 35 widely popular app activities (ranging from social networking and dating to personal health and presidential campaigns) shows that NetScope yields high detection accuracy (78.04% precision and 76.04% recall on average).