This study explores various measures of the ethnic makeup in a classroom and their relationship with student outcomes. We examine whether measures of ethnic diversity are related to achievement (mathematics, reading) and feeling of belonging with one's peers over and above commonly investigated composition characteristics. Multilevel analyses were based on data from a representative sample of 18,762 elementary school students in 903 classrooms. The proportion of minority students and diversity measures showed negative associations with student outcomes in separate models. Including diversity measures and the proportion of minority students, diversity of minority students mostly lost its significance. However, the results suggest that diversity measures may provide additional information over and above other classroom characteristics for some student outcomes. The various measures of diversity led to comparable results. Educational Impact and Implications Statement This study suggests that the ethnic makeup of classrooms is related to student outcomes. That is, students in classes with a higher proportion of ethnic minority students showed slightly lower achievement and feeling of belonging with one's peers even if the socioeconomic status, the immigrant background of the family, cognitive ability, and gender of the student is equal. In addition to the proportion of ethnic minority students, average socioeconomic status, and average cognitive abilities, we looked at the ethnic heterogeneity in each classroom and found that this was mostly independent from student outcomes. Only for math we found a positive association indicating that students in a more ethnically diverse classroom showed slightly higher test scores-however, this slight association cannot be interpreted as a causal relationship because of our cross-sectional design. The findings suggest that measures of heterogeneity may uncover relationships that the mere proportion of minority students which disregards various ethnic groups in the classroom is unable to show and open a discussion on how to investigate effects of ethnic diversity in educational research.