Dealing with ethnic and linguistic diversity is one of the major challenges in today's education. Therefore, the formulation of competencies for teachers and teacher training should take into account the specific requirements of teaching in multicultural schools. In 2002, a series of small-scale studies were conducted to identify and formulate teachers' communicative competencies in multicultural classes through a mixture of instruments and data sources. The studies aimed at providing recommendations for improving these competencies through teacher education and professional development. This study reports on the multiple instruments used to study the practical knowledge and behaviour of experienced teachers in multicultural schools by discussing research evidence from a series of case studies from primary and secondary education. These case studies explored teachers' knowledge, beliefs and behaviours in creating healthy teacher-student relationships as well as making lesson content accessible for all students, including students with limited language skills. The findings show how small-scale studies with multiple data collection techniques can help in providing an empirical foundation for the formulation and specification of teacher competencies in multicultural settings. Based on our findings, we argue that by conducting multiple investigations in different settings, using a variety of instruments and by interpreting this data from different communicative domains, a valid, reliable and complete picture of teachers' scientific competencies can be obtained. (Contains 2 tables.)