Full text can be accessed at http://www.tandfonline.com/doi/abs/10.1080/17538947.2013.782073Transparency and Accountability (T&A) interventions are emergent social technologies in middle and low-income countries. They bring together citizen sensors, mobile communications, geo-browsers and social organization to raise public awareness on the extent of governance deficits, and monitor government's (in)action. Due to their novelty, almost all we know about the effectiveness of T&A interventions comes from gray literature. Can citizen sensors radically increase the transparency of the state, or are changes brought about by T&A interventions more likely to be incremental? We review the literature on transparency policies and describe their drivers, characteristics and supply–demand dynamics. We discuss promising cases of T&A interventions in East Africa, the empirical focus of an on-going collaborative research program. We conclude that the effect of T&A interventions is more likely to be incremental and mediated by existing organizations and professional users who populate the space between the state and citizens. Two elements at the interface between supply and demand seem rather crucial for designers of T&A interventions: accountability-relevant data and extreme publics.