Recent years have witnessed considerable enthusiasm over open data. Several studies have documented its potential to spur economic innovation and social transformation, and to usher in fresh forms of political and government accountability. Yet for all the enthusiasm, we know little about how open data actually works, and what forms of impact it is really having. This report seeks to remedy that informational shortcoming. Supported by Omidyar Network, the GovLab has conducted 19 detailed case studies of open data projects around the world. The case studies were selected for their sectoral and geographic representativeness. They were built in part from secondary sources (“desk research”), but also from a number of first-hand interviews with important players and key stakeholders. They are presented at length, in narrative format, on an online repository, Open Data’s Impact (odimpact.org). In this paper, we consider some overarching lessons that can be learned from the case studies and assemble them within an analytical framework that can help us better understand what works, and what doesn’t, when it comes to open data.