A software component must be tested every time it is reused, to guarantee the quality of both the component itself and the system in which it is to be integrated. To reduce testing costs, we propose a model to build highly testable components by embedding testing and monitoring mechanisms inside them. The approach is useful to component developers, who can use these built-in test capabilities in the testing phase, as well as to component users, who can verify whether the component fulfills its contract. Our approach proposes the insertion of built-in mechanisms directly into intermediate code, allowing its use even in COTS components.