In the context of interoperable public services, the concept of openness mainly relates to data, specifications and software. Open government data (here simply referred ‘open data’) refers to the idea that all public data should be freely available for use and reuse by others, unless restrictions apply e.g. for protection of personal data, confidentiality, or intellectual property rights. Public administrations collect and generate huge amounts of data. This data should be published with as few restrictions as possible and clear licences for its use to allow better scrutiny of administrations’ decision-making processes and realise transparency in practice. The use of open source software technologies and products can help save development cost, avoid a lock-in effect and allow fast adaptation to specific business needs because the developer communities that support them are constantly adapting them. Public administrations should not only use open source software but whenever possible contribute to the pertinent developer communities. Open source is an enabler of the underlying principle on reusability. The level of openness of a specification/standard is decisive for the reuse of software components implementing that specification. This also applies when such components are used to introduce new public services. If the openness principle applies in full: * all stakeholders have the opportunity to contribute to the development of the specification and a public review is part of the decision-making process; * the specification is available for everyone to study; * intellectual property rights to the specification are licensed in a way that allows implementation in both proprietary and open source software, and preferably on a royalty-free basis. Due to their positive effect on interoperability, the use of open specifications has been promoted in many policy statements and is encouraged for public service delivery. The positive effect of open specifications is demonstrated by the internet ecosystem. However, public administrations may decide to use less open specifications if open ones do not exist or do not meet functional needs. In all cases, specifications should be mature and sufficiently supported by the market, unless they are being used to create innovative solutions. Lastly, openness also means empowering citizens and businesses to get involved in the design of new services, to contribute to service improvement and to give feedback about the quality of the existing public services.