The object of this article is the structure and evolution of skills developed by pupils at primary level. Starting from an analysis of the panel data provided by the French Ministry of Education, the main object of this paper is an original measurement of skills using structural models. The findings of this research raise two complementary issues. The first issue concerns the cross-curricular nature of skills suggesting a logic of disciplinary interdependence. The acquisition of a skill may be strongly correlated to the acquisition of other skills belonging to the same subject area or to other subject areas. The notion of a set of skills is used to account for the connections between the different aspects of acquired skills. The second issue concerns the evolution of skills over time. Here the aim will be to identify the kinds of skills that are the most predictive of subsequent educational success. This issue is highly relevant for educational policy-making. It is hoped that the results presented in this paper will improve our understanding of the ways in which schools might provide pupils with the best chances of success in the earliest stages of their educational career.