An approach based on competences is gaining momentum in several French-speaking countries. This approach, which insists on assembling acquisitions for solving complex problems, features two paradoxes: It increases the exigency level in systems which aim at fighting school failure or backwardness. It asks teachers to educate competent students though means to reach that goal remain hard to understand and mysterious. In front of those two paradoxes, the trust-worthiness of the approch based on competences lies on a new way of considering the evaluation of acquisitions. How to evaluate competences while taking into account both paradoxes? That’s the question that this aticle aims at answering.