One of the major goals of the European Long-Term Ecological Research (eLTER) and the up-coming eLTER Research Infrastructure (eLTER RI) is to provide reliable and quality-controlled long-term data for scientific analysis as well as the assessment of environmental policy impacts. For this purpose, eLTER has designed, implemented and operates a federated data infrastruc-ture called the eLTER Information System. This e-infrastructure offers data stored in existing partner data systems, harmonised by a central discovery portal and federated data access com-ponents providing a common information management infrastructure for making environmental data available from distributed resources provided by the contributing LTER national networks. Designing, building and optimising such a pan-European environmental data infrastructure is a lengthy and complex process that is based on a set of criteria defined by user needs, share-holder requirements and general service and technology best practises. To further improve and extend the eLTER Information System, user needs have recently been collected by (a) targeted interviews with selected stakeholders to identify the scope and background of the data and ICT requirements, (b) workshops mapping user requirements based on personas derived from the interviews, and (c) analysis work on extracting so-called user stories. The requirements collec-tions are used to derive functional (i.e. the behaviour of essential features of the system) and non-functional (i.e. the general characteristics of the system) requirements for the IT infrastruc-ture and services. These collected requirements revolve around the development of workflows for the ingestion, curation and publication of data objects including the creation, harvesting, discovery and visualisation of metadata as well as providing means to support the analysis of these datasets and communicating study results. This presentation will provide an overview of the current stage of the data infrastructure as well as its major components, provide an outlook for future developments and discuss the technical and scientific challenges of building the eLTER Information System.