European Union (EU) Member States are currently in the implementation phase of the INSPIRE Directive, building a cross-border spatial data infrastructure (SDI) for Europe by adding components to their existing spatial data-sharing resources. Although INSPIRE’s technical guidelines have been based on existing international standards, implementations often use standards either in different ways or different versions. In addition, standards themselves regularly evolve with often limited coordination between changes in different standards. Such issues can limit interoperability between information systems, especially in cross-border contexts. There is a need for additional guidance and increased collaboration to share best practices and approaches in INSPIRE’s implementation. Moreover, technical components (software, technical guidelines, ‘semantic assets’ etc.) need to be referenced and shared to support the EU Member States and address these interoperability issues. Such a resource will also help other European policy areas to reuse this material when looking to share and integrate spatial in their, broadly speaking, e-government activities. This includes contributing to technologies and infrastructures to support open data and the EU’s key ICT policy, the Digital Agenda for Europe. The EU’s Interoperability Solutions for European Public Administrations (ISA) Programme is an initiative covering many aspects of the interoperability requirements for cross-border and cross-sector e-government, from strategic policy needs in legal and organisational interoperability through to semantic and technical requirements. Under ISA, a Reusable INSPIRE Reference Platform (ARE3NA) is addressing the dual needs to support interoperability between Member States’ implementations and the reuse of INSPIRE components in other sectors. Based in the Unit responsible for the technical coordination of INSPIRE at the European Commission’s (EC) Joint Research Centre (JRC), the ARE3NA team have already established several baseline inventories as part of its initial evidence base. This includes an inventory of European policies using spatial information and a related inventory of data-sharing platforms and tools. Another inventory has reviewed recent studies to identifying components already being used in Member States, including associated architecture documents and the technologies being used in some INSPIRE implementation tasks. These have, in turn helped to draft generic workflows in the INSPIRE implementation process. In many instances the tools being used are Free and Open Source Software (FOSS), and a third inventory has both built on the recent work of Steiniger and Hunter (2012) to create an overview of the current groups of technologies being deployed in Member States to support INSPIRE. In order to provide more reference components, work has also created inventories of the standards being used in INSPIRE, with a final inventory providing a baseline position on the amount of geospatial data current being shared in national open data portals. The online ARE3NA platform space to share these findings has recently been established within the ISA programme’s JoinUp platform, a resource to support developers and e-government professionals share their experience with interoperability solutions. This platform will help users to validate the material being found and to gather further examples towards a ‘constellation’ of components connected to INSPIRE’s legal items, implementation tasks and the technologies (etc.) involved. The platform is being supported by a survey to gather further evidence that will run for the duration of the project. The survey aims to establish what other technologies are being used to support INSPIRE implementation and where there may be ‘missing components’ or gaps in current technologies or standards or, similarly, gaps between the data-sharing infrastructure of INSPIRE and the needs of European cross-border and cross-sector e-government, including those that ARE3NA can help to address by supporting developments in new or existing open source projects. The survey also aims to identify good practices and training to support INSPIRE implementation across the Member States in the coming years. Already, ARE3NA has addressed one gap by developing the open source INSPIRE Registry. This fundamental component for geospatial data interoperability across European SDIs provides a means to manage and share codelists from INSPIRE data models. By developing the INSPIRE Registry with ISA in mind, these codes can also be readily adopted in other contexts, potentially reducing multiple vocabularies for similar spatial objects across Europe. By basing its development on open source software tools, the INSPIRE Registry itself becomes a reusable component in JoinUp, so that other interoperability projects can adopt the software for their own purposes, including any register, including those involving hierarchical information, from organogram details to thesauri. Following the collaborative processes seen throughout INSPIRE, the presentation will provide an opportunity to introduce ARE3NA to a key stakeholder audience, with an invitation to participate in the platform and survey as well as present the evidence to date, illustrating the role of FOSS in establishing interoperable SDIs in Europe.