This study examines the impact of private school attendance on segregation and student achievement in compulsory school in Denmark. We show that increased private school attendance is driven by students from high socio-economic groups. Leveraging variation across municipalities, grade and calendar years and instrumental variables based on private school openings, we find that higher private school enrollment is associated with higher segregation of disadvantaged children. From event study models of the private school openings and a mover design that controls for student parental background, peer parental background, past achievement and non-cognitive scores, we find small achievement effects of private school attendance.