The last two decades have witnessed a dramatic increase in the use of patent citation data in social science research. Facilitated by the digitization of patent data and increasing computing power, a community of practice has developed that has identified methods for using these data to measure attributes of innovations such as impact and originality; trace flows of knowledge across individuals, institutions and regions; and map innovation networks. The objectives of this chapter are threefold. First, it takes stock of these main uses. Second, it discusses four pitfalls associated with patent citation data, relating to office, time and technology, examiner, and strategic effects. Third, it highlights gaps in our understanding and offers directions for future research.