CLIL programmes are firmly established in German schools. While empirical studies have provided insights into aspects such as language- and content-related achievement, motivation, and cultural learning, little is known about classroom practices and students' perceptions thereof. Using ADiBE's instruments, this mixed-method study explores how teachers and students view differentiation and diversity-sensitive CLIL classroom practices in North Rhine-Westphalia, Germany. In total, 31 teachers and 595 students shared their experiences concerning methods, materials, classroom arrangements, scaffolding, and differentiation techniques used to foster the educational success of linguistically and academically diverse students. A classroom observation (45 minutes) geography lesson and an accompanying focus-group interview with six students allowed for deeper insights into specific practices and how they were perceived. The results show (how) diversity is frequently taken into account in various ways, yet there are also substantial challenges in catering to a diverse student population. The findings of this first phase of the ADiBE project lay the groundwork for further research on heterogeneity in CLIL and will be used as a stepping stone for the creation of research-informed CLIL teaching materials and teacher-training resources.