Group composition effects in educationHow the school context impacts the way students and teachers feel, achieve, and develop The school context affects the lives of all members: students and staff members. One of the major school context variables that has been studied in educational effectiveness research in thepast few decades, was the group composition. The group composition effect can be defined as the impact of students or teachers aggregated characteristics (SES, ethnic background, prior achievement, gender, self-efficacy beliefs etc.) by taking into account the individual-level variables. In this dissertation project, two types of school composition effects are central: the composition of a schools student body and the composition of a schools teacher team. More specifically, the impact of these two types of school composition on students and teachers psychosocial and cognitive functioning is investigated. The measurement of group composition effects has been the focus of several methodological discussions. The methodological flaws in current group composition research mainly lie in the cross-sectional nature of most compositional studies, the under-specification of the level-1 predictors included in the model, and the way in which both the level-1 and level-2 variables are measured. In an attempt to tackle these shortcomings, the current project applies two rather new methodologies, namely piecewise growth curve modeling and doubly latent modeling. These new methodologies will lead to more valid and reliable results.The present PhD project consists of five distinct studies. The first study concerns the effect of class composition by gender and ability on secondary school students school well-being and academic self-concept. The second study focuses on the impact of pupil socio-ethnic background and school composition on pupils language growth. The third study addresses the effect of collective teacher efficacy beliefs on teachers job satisfaction, stress, and self-efficacy. The fourth study investigates the impact of aclass student body on teachers psychosocial functioning. Finally, thefifth study explores the impact of a schools teacher teams collective efficacy beliefs on student achievement growth. The results of this dissertation project will feed the debate on group composition effects and are relevant to policy makers and school leaders who need scientific evidence regarding school composition and its effect.