In the 1970s W. Doise, G. Mugny and A.-N. Perret-Clermont underlined for the first time the essential role played by social interactions in cognitive development. Since then, many authors have been studying social interactions and their mediating role in knowledge apprehension and in skills acquisition. Inspired by L. Vygotsky's theory, many contextualized researches were conducted that showed social interactions, namely peer interactions, were a main facilitator factor for pupils' socio-cognitive development, both in performing their math tasks and in relation to their overall academic achievement in this subject. The contextualized studies also underlined the power of peer interactions in promoting pupils' social integration and participation. Interaction and Knowledge is a research-action project that was implemented in several math classes (5th to 11th grade) with the goal of promoting peer interactions in math classes as a way of changing the didactic contract and facilitating pupils' socialization and school achievement. A deep analysis of peer interactions shows how important the social and cultural aspects of learning mathematics are in pupils' performances in math class. The examples discussed underline the role of many psycho-social factors such as the situation, the social and academic status of the peer, the work instructions that are given, and the didactic contract. The data stress the importance of this kind of analysis if the goal is to promote more positive attitudes toward math and better school performance in this area. (Author/YDS)