The research is designed for the diagnostics of the existing teachers’ argumentation skills and their development. The analysis of intermediate WP7 materials and other scientific literature sources highlighted the structure, types of arguments and the model of argumentative competence. The complex case study of the development of teachers’ argumentation skills is designed around three major argumentation performance lines: A line is teachers’ personal line, revealing argumentation skills while performing personal action research connected to the voluntary chosen research question; B line – preparation of final thesis, which is related to the quality of argumentation while working with the research focus group and scientific adviser; C line – the defence of final thesis which investigates the teachers’ argumentation performance during the procedure of final thesis defence. The data of the investigation allow stating that A line arguments may be characterised as linear arguments, prevailing in the simple discussion and classroom debates. B line context already demonstrates teachers’ abilities to produce scientific arguments which are later polished and defended in C line. After the defence teachers also state that they acquired the competence to present multifunctional argumentative discourse for their pupils/ students, for the written scientific text and for the oral public presentation.