How to evaluate competencies? But also, what are exactly competencies? According to classical definition, their are related to categories of situations. But this concept has never been operationalised. When children are confronted with a complex problem to solve, is it still relevant to conserve the concept of competencies and, if so, with what meaning? In agreement with the ideas proposed by Brown, Collins & Duguid (1989), but also by Resnick & Klopfer (1989) and all the cognitive studies on problem solving, we hypothesize that the mastery of the adequate procedure is not sufficient for solving complex problems in which the mobilization of this procedure is requested. The ability of the children to construct an adequate representation of the problem, but also their aptitude to cope with the knowledge overload are crucial. The empirical study presented in this paper has been designed in order to document this hypothesis. Some mathematical concepts (measurement of distance and volume, "traduction d’échelles", estimation of time) have been assessed in various contexts with a large sample of 11/12 years-old Belgian children (N = 1 436). Some parts of the assessment test are related to declarative knowledge ; other parts to procedural knowledge. The reported investigation indicates that some complex procedural abilities are mastered by some pupils, but not used in situation in which it is relevant to articulate some of these operations. These findings are discussed in regard of the difficult issue of competencies assessment, but also in regard of the mobilization of meta-cognitive regulation. Finally, the authors argue in favor of the assessment of the meta-cognitive process as an integrated component of the competencies evaluation.