A growing part of the public is concerned about cities being designed and governed in a responsible way. In the contemporary information society, however, the democratic obligation of the citizens to inform themselves thoroughly, so that they can participate in public affairs has become impossible to fulfill. Rather than submitting to the opinions of self-proclaimed experts, citizens need new ways to make sense of what is going on around them. Accountability technologies stand for new innovative approaches to bottom-up governance: technologies to monitor those in power and hold them accountable for their actions. Accountability technologies are designed to coordinate citizen-led data collection, visualization and analysis in order to achieve social change. This book takes a close look at initiatives that have succeeded in making an impact on the reality of the city, as well as the motivations, strategies and tactics of the people who create and use these technologies. How can data generated by citizens be put into action?