This editorial explores the potential of social media and mobile technologies to foster citizen engagement and participation in urban planning. We argue that there is a lot of wishful thinking, but little empirically validated knowledge in this emerging field of study. We outline key developments and pay attention to larger societal and political trends. The aim of this special issue is: 1) To offer a critical state-of-the-art overview of empirical research; and 2) to explore whether social media and mobile technologies have measurable effects on citizens’ engagement beyond traditional mobilization and participation tools.