Heterogeneity is part of everyday life, and classrooms mirror this reality. Thus, students’ broad array of learning needs should, as stipulated in numerous publicly accessible documents, such as school laws, school regulations and standards for teacher training, be meaningfully addressed through means of differentiated instruction. The present doctoral thesis examines how differentiated instruction at the micro level is implemented in school practice. In particular, teachers’ differentiated practice in terms of frequency of use, as well as context variables are examined. Data analyses from a sample of N = 295 German (as a school subject) and/or English teachers from different school tracks, indicated that: a) differentiated instruction is scarcely used in daily practice, b) that German and English teachers hold a low invariance in their differentiated instructional practices, and c) that in comparison to low and comprehensive school track teachers, high track school teachers implement far less differentiated instruction in their in-class practice. Additionally, the analyses from the present doctoral thesis show that teachers’ implementation of differentiated instruction is dependent on context factors, such as teacher collaboration in planning and implementation of lessons, the perceived quality of teacher training with regard to dealing with heterogeneity as well as teachers’ willingness to implement differentiated instruction, their attitudes and expectations of self-efficacy. Lastly, post-hoc analysis showed, with regard to the frequency of use, links between teachers’ personality and the school track at which they teach. Given that the results stem from school practice, they provide practical implications, such as information on the importance and necessity of increasing the quality of teacher training. Further practical implications are explored and future lines of research are discussed.