The data was collected for the TRIAL: TRansitions Into Active Living study funded by NWO (grant no.: NWA.1160.18.249). The original aim of the this study was to examine the influence of the life transition ‘from primary to secondary education’ on physical activities and sports. In wave 1 (this codebook) the participants just started secondary school. In the survey they were asked about their sport participation in the final grade of primary education, their current sport participation and physical activity, and additional variables. In addition, participants completed a motor skill test and their weight, standing height and sitting height were measured.