The evaluation of mediation actions is facing the hagiographic pitfall and the objectivist horizon of measuring its effects. It is best understood less as a control or verification operation of the adequacy between objectives and results than as a space for the production of value. The article begins with a reflection on evaluation issues and how the evaluation of mediation questions evaluation practices. It continues with the presentation of five studies carried out in recent years in France on the cultural mediation of music in a school context from different musical institutions (opera, orchestra, etc.). It appears that the evaluation of the mediation schemes’ effects is all the more fruitful as the method adopted is at the same time continuous, participative and recursive. With this methodological trio, the evaluation of mediation becomes a mediation tool and a common workspace of meaning.