This article discusses some of the main conceptual parameters of freedom of information (FOI) theory, before reviewing the available evidence from a range of studies. There is confusion about what kind of right FOI is (civil, political, social), and what consequences can be expected from an FOI regime, particularly effects on governance, corruption, and the investment climate. The article presents cases studies of civil society activism on FOI in India and South Africa, concluding that the extent to which FOI laws have a substantive impact depends on the level of citizen engagement in the use of the law. The article reviews a number of approaches for assessing the impact of FOI, including approaches which study the mutually reinforcing ‘supply’ and ‘demand’ for transparency, and the role of civil society. It concludes that the academic community and the FOI community of practice need to come together to devise robust and rigorous methodologies for impact assessment.