Despite its benefits, the adoption of open source by public administrations across Europe has not historically been a sustainable journey. This has led the European Commission’s Open Source Observatory (OSOR) to investigate factors behind the sustainability of public sector open source communities in order to produce dedicated Guidelines on this topic. The Guidelines offer useful tips and step-by-step recommendations for civil servants at all administrative levels, project managers, IT developers, and open source software (OSS) enthusiasts looking to engage with OSS. The purpose of the Guidelines is to act as a practical tool that can be used by public sector officials interested in establishing or joining open source communities as well as by members of such communities. We collected primary data through a comprehensive survey, five in-depth case studies, and secondary material. The Guidelines are based on the five key lessons identified through our research that can help public sector officials to engage with and sustain open source communities.