In both developed and developing societies, there has been increasing interest in recent years in the potential of transparency — the provision of information to the public — to improve governance. In this article, we characterize and assess the evolution of transparency from an end in itself to a tool for resolving increasingly practical concerns of governance and government performance. After delineating four distinct varieties of transparency, we then focus on the type variety that has received the most rigorous empirical scrutiny from social scientists—so-called “transparency and accountability” (T/A) interventions intended to improve the quality of public services and governance in developing countries. T/A interventions have yielded mixed results: some are highly successful; others appear to have little impact. We develop a rubric of five ideal typical “worlds” facing transparency that helps to account for this variation in outcomes. Reform based on transparency can face obstacles of collective action, political resistance, and long implementation chains. T/A interventions are more likely to succeed in contextual “worlds” with fewer of these obstacles. We find that sixteen experimental evaluations of T/A interventions are consistent with the theoretical predictions of our five-worlds rubric.