As most studies focus on the outcomes of multiculturalism, assimilation and colourblindness (MAC) for students, little is known about why schools vary in the diversity approaches that they adopt and the role of the teacher in this process. This study explores how teachers implement MAC diversity approaches and what drives them in doing so. Based on a cluster analysis of 38 schools, three extreme cases were selected that either valued (multiculturalism), rejected (assimilation) or ignored (colourblindness) ethnic diversity. In these schools observations and in-depth interviews were conducted with 39 teachers, 23 students and 3 headteachers. Our results indicate that teachers varied in their implementation of MAC diversity approaches according to four diversity domains: 1) multilingualism, 2) religious diversity, 3) ethnic discrimination and 4) diversity curriculum. Teachers' beliefs and attitudes (individual-level) and their preconditions for teaching (micro-level) appeared as the most important motivators for adopting a particular diversity approach.