Open data is data that can be accessed, shared, used and reused without any barrier for any type of (re)user. According to the Open Definition, open data can be defined as data that be freely used, modified, and shared by anyone for any purpose subject, at most, to measures that preserve provenance and openness. Open data requires datasets to be either in the public domain, or distributed through an open license. The data must be provided as a whole, free of charge, and preferably downloadable via the Internet, including any additional information that might be necessary to comply with the open license’s terms. Openness requires the data to be provided in a readily machine-readable form. The format must be open as well, meaning that it does not place any restriction upon its use, and that the files in that format can be processed with open-source software tools. The Open Definition speaks broadly of open ‘works’, rather than of open data. Focusing on data tout court, one can move from the Open Government Data (OGD) principles. According to the OGD principles, which are arguably foundational in understanding the concept of open data, data must be: Complete; Primary; Timely; Accessible; Machine-processable; Non-discriminatory; Non-proprietary; and License-free. Compliance with the OGD principles needs to be demonstrable, i.e. there need to be accountability measures in place to allow the review of the adherence to the principles above. The concepts of Open Work and open data highlight how data needs to be both legally, technically and financially open, so either in the public domain or covered by an open license, and kept in a machine-readable and non-proprietary format. Open data aims at making information available to everybody, for any purpose, in a machine-readable and interoperable format, based on open standards and digestible by free/libre open source software (FLOSS). Also with respect to the financial accessibility open data is data available free of charge. Marginal costs of dissemination are accepted by some as a reasonable cost for users. However, open data is data that can be accessed and reused without any barrier for any type of reuse, and some user groups experience any price to be paid as a barrier.