The paper contributes to research on component and multi- agent systems by presenting a practical approach to the development of a modular and reusable middleware. In particular, we address the problem of the construction of the core of a middleware for MAS. Then we introduce two case studies for two different application domains: bi- ological data integration and quality assurance in manufacturing. Our experience proves that the component-based approach provides several benefits, such as the facilitation of refactoring and reusability, but also introduces some pitfalls, such as excessive reuse. We acknowledge that reuse exploits its potential in the lower layers of a system, because com- ponents are quasi-free of business level concepts.