Recent developments in mobile technologies have produced a new kind of device: a programmable mobile phone, the smartphone. In this article, the authors argue that the technological and social characteristics of this device make it a useful tool in social sciences, particularly sociology, social psychology, urban studies, technology assessment, and media studies. The device is willingly carried by a large fraction of people in developed countries, integrates a number of technologies for automatic observation, can be programmed to interact with the user, and can communicate with remote researchers. This allows unobtrusive and cost-effective access to previously inaccessible sources of data on everyday social behavior, such as physical proximity of people, phone calls, and patterns of movement. The authors describe three studies in human behavior that have augmented existing methods with the smartphone, two of which the authors conducted themselves. Based on their experience, the authors critically evaluate the improvements and threats to validity and reliability of smartphone-augmented methods. These approaches are rapidly becoming feasible for the social scientist, since research software for smartphones have been published in open source, which lowers the technical and economic investment needed for their utilization in research.