This study aims to give an overview of school segregation in Stockholm and its development during the period 2000 to 2010. Further, it aims to examine the effects of school segregation on student achievement. The first part of the study uses register data to measure segregation in schools from 2000 to 2010, the second part utilizes the Stockholm School Survey 2010 for measuring student demographics and school achievement. The examination of school segregation in Stockholm reveal a substantial segregation between schools, on both levels of non-native background and parental education. The trends have been stable for segregation on non-native background and decreased slightly for parental education from 2000 to 2010, however, they both remain at relatively high levels. Multilevel analysis show that student school achievement is negatively impacted by increased concentration of students with disadvantaged characteristics, i.e. higher levels of students with non-native background and lower levels of parental education. The results also indicate that non-native students are more negatively affected by these effects. Further, the analysis tests for threshold effects of segregation, but no such effects can be identified and it seems to be more or less linear, higher degree of segregation leads to stronger effects. It is concluded that differences between schools have an unequal and unfair effect on student school achievement.