Part 3: Reference Architectures and Patterns International audience The Once-Only Principle states that citizens and businesses provide data only once in contact with public administrations. So far, many European countries have started to implement the Once-Only Principle at national level, but its cross-border implementation is still fragmented and limited. This paper presents the development of a Reference Architecture for the Once-Only Principle in Europe. The case study, stemming from the EU-funded Once-Only Principle project (TOOP) highlights the challenges faced by the architecture team when developing the Reference Architecture that tackles the Once-Only Principle across different countries and policy domains. The architecture is not built from scratch, but re-uses and enhances already available building blocks in order to seamlessly preserve interoperability and to comply with regulations and existing technical standards, leaving at the same time enough space for vendors and open source developers to propose their compliant solutions, whatever is their business model.