Interoperability is one of the key challenges in present and future software-intensive systems that are large, distributed, and increasingly built as integration of existing and third parties components or systems, of legacy parts, and of newly developed parts. Moreover, such systems evolve over time due to different reasons, e.g., features are added, changed or removed, new protocols are supported, standards are changed, refactoring.To help large companies identifying how to manage and improve interoperability among their evolving software systems, our objective is to develop an interoperability model for large systems by focusing on software development.Our method to conceive and evaluate the model is through a tight collaboration among two universities and five large international companies.The results of our work are the INTERO model and the INTERO evaluation framework. They permit to analyse the specific interoperability problem, to conceive strategies to enhance interoperability, and finally to re-evaluate the problem in order to understand whether there is an improvement in terms of software interoperability.