Interoperability is a key prerequisite for efficient cross-agency administrative processes and the implementation of the once-only principle. It is considered across four layers: legal, organisational, semantic, and technical. The EU's European Interoperability Framework (EIF) provides the conceptual framework. In Germany, the IT Planning Council coordinates interoperability efforts between the federal and state levels, for example through maintaining the XÖV standards ecosystem for data exchange in the public sector. Open standards, shared data models, and API strategies are essential enablers. Lack of interoperability leads to duplicate data entry, delays, and increased costs, making it a central obstacle to administrative digitalisation.