This paper addresses the current trends and issues with regards to opening up data held by public entities in various sectors, including public sector information, geographic data, cultural heritage, scientific publications and data. In the paper, opening up public data is defined as making it available for any purpose of use. While several initiatives have been taken within Europe to make public data available, many issues still remain unsolved. Based on the state of play in various sectors, this paper gives an overview of common issues that need to be addressed in order to move to more and better accessibility and reusability of public data. It will argue that even if sectors are currently regulated by different laws and policies governing data of a different nature, a common techno-legal framework can be defined to address legal, cultural and institutional challenges in a cross-sectorial manner.