The “Once-Only” Principle Project (TOOP) explores and demonstrates the “once-only” principle through multiple sustainable pilots, using a federated architecture on a cross-border collaborative pan-European scale in order to identify drivers and barriers and to provide a basis for future implementations and wider use. Three pilots are implemented: (1) Cross-border e-Services for Business Mobility, (2) Updating Connected Company Data and (3) Online Ship and Crew Certificates. TOOP has the ambition to connect 59 information systems from 21 countries. The methodological approach of TOOP is based on an exploratory and agile pilot life-cycle approach to cross-border pilots, the planning and implementation of which is supported by the development of a generic federated architecture and building blocks, the identification and mitigation of barriers, including legal issues, and the evaluation of the results, with pro-active dissemination and sustainable exploitation of the results throughout the project. TOOP’s main technological innovation is a generic federated OOP architecture that supports the interconnection and interoperability of national registries at the EU level. The pilots planned are ambitious as they contain innovations and provide a basis for recommendations for future implementations and the wider use of the OOP. The project is an important learning environment for OOP in particular and European e-Government in general. The pilots are scalable and can be extended to all Member States in subsequent years. TOOP’s consortia of 51 organisations consists of 19 national administrations (or authorized thereby) from 19 different EU Member States and 2 Associated Countries. The public administrations provide effective piloting, research partners make sure that the key preconditions for effective piloting are met, and private entities support exploitation, all complementing each other in the whole range of activities and areas dealt with in TOOP.