The paper brings the results of qualitative analysis of teacher expertise realized within the project Expert Teacher: the nature of expertise and determinants of professional development (in FLT perspective). The project designed as a multiple case study is aimed at systematic analysis of the nature of foreign language teacher expertise and the development of its empirical model. During the first phase carried out in 2011 the data were collected in interviews with 30 teachers who had been nominated as expert teachers of English or German languages by their headmasters or teacher trainers from universities. Semi-structured interviews followed the observations of the teachers in their classrooms and focused on the aspects of expertise that were not directly observable, namely the teachers’ knowledge of the subject matter, their knowledge of the pupils, pedagogical content knowledge, ability to analyse and interpret educational phenomena, justify their actions and decisions, formulate their subjective theories, and pursue their professional development. The results revealed some interesting aspects of expertise of foreign language teachers.