In the "transition from PRIMary to Secondary education (PRIMS)" project we collected data to supplement the Netherlands Cohort Study on Education (NCO, Nationaal Cohortonderzoek Onderwijs, for more information see Haelermans et al. 2020). The PRIMS project was funded by the Nationaal Regieorgaan Onderwijs (NRO) and carried out by the University of Groningen (RUG) and the University of Amsterdam (UvA), in collaboration with NCO. PRIMS aims to study the role of peers in the transition from primary to secondary school. More specifically, we want to shed light on educational inequalities in the transition from primary to secondary education, as well as students’ social integration during this school transition. Using the PRIMS data set, we can address questions on the influence of peers on educational decisions, aspirations, and performance, as well as the impact of positive and negative peer relationships on school well-being and academic achievement.