This introductory article draws on the contributions to this special issue to consider the implications of a transparency turn in global environmental and sustainability governance. Three interrelated aspects are addressed: why transparency now? How is transparency being institutionalized? And what effects does it have? In analyzing the spread of transparency in governance, the article highlights the broader (contested) normative context that shapes both its embrace by various actors and its institutionalization. I argue that the effects of transparency-whether it informs, empowers or improves environmental performance-remain uneven, with transparency falling short of meeting the ends many anticipate from it. Nonetheless, as the contributions to this issue make clear, transparency has indeed come of age as a defining feature of our current and future politics. (c) 2010 by the Massachusetts Institute of Technology.