Literature and policy reports suggest that open data can be used as a tool to enhance transparency. However, several researchers have challenged the idea that the release of government data will result in increased transparency and the idea that transparency automatically leads to more trust in the government. Moreover, transparency may have undesired effects. Limited research has been conducted on these sometimes conflicting findings and challenges, and on how open data policies should deal with this. It is not clear what encompasses an effective open data policy and how this influences transparency. The papers in this special issue contribute to this area. In the introduction of this special issue we present the state-of-the-art with regard to definitions, developments, research, challenges and barriers related to open data transparency and open data policies. Finally, a summary of the papers included in this special issue is provided.