This thesis analyzes the effect of between-school and within-school segregation on educational inequalities in secondary education in Geneva. From the SRED (Service of education research) school database and the PISA 2003 survey, the aim is first to assess the extent of school segregation in Geneva and to identify if it is rather built between schools or within schools. Next, I analyze the mechanisms that cause segregation to determine whether it is related to urban segregation or whether it is related to the education system, which separates students in different tracks. Finally, I estimate the consequences of segregation on educational inequalities using multilevel models. The goal is to assess whether between-school and within-school segregation has a specific effect on students' level of competence at the end of compulsory education, on the training program (general or vocational) followed in post-compulsory education and on student engagement at school.