This study presents a collective case study design and mixed method triangulation as a method for research on teachers’ knowledge and teachers’ classroom practice, in particular the study of how in-service teachers view their teaching, and how teachers use practical knowledge and theories during teaching in the classroom. The data collected, the categories and the interpretations were confronted using a mixed method triangulation procedure, with the aid of software for quality analysis (ATLAS.ti). The relationship between teachers’ personal practice and theories on teaching offers a knowledge and understanding of some teachers’ vision about own teaching, formal and informal learning of teachers and how teachers use practical knowledge and theories during teaching in the classroom. The research offers some useful suggestions about the methodological contribution for the retrieval and the analysis of qualitative data with a mixed method approach.