Interoperability, the ability of different technological applications to exchange data, is viewed by many as an important goal for eHealth, as it can save money and improve the quality of care and patient safety. However, creating an interoperable infrastructure for eHealth is a difficult task. In this paper, we present a maturity model that aids eHealth developers to determine what level of interoperability they should strive for, and that allows researchers to benchmark interoperable eHealth infrastructures in terms of maturity. For each level in the model, we illustrate what the interoperable infrastructure looks like from the technical point of view, we list implications for working procedures and we discuss the role of standardization. The maturity model has five levels. At level 0, there is no interoperability: The eHealth application is a silo. At level 1, Peer-to-peer systems, single applications are linked for simple data exchange. At level 2, Distributed systems, multiple applications are linked to achieve a common objective. At level 3, Integrated systems, applications from different suppliers are linked in a closed infrastructure. And at level 4, Universal interoperability, finally, applications are linked in an open infrastructure from which everybody is free to (dis)connect. We demonstrate the application of the maturity model via the case of an interoperable eHealth infrastructure for primary care. Reaching the most technically advanced form of interoperability (level 4) is not a goal eHealth developers should always strive for. They should set their goal with regard to the desired interoperability level for their situation and should then determine what they should do in terms of technique, working procedures, and standardization.