LAUSR

Abstract In this exploratory study, we analyzed mentor teachers' (MTs') reasoning about whether, when, and how to intervene during student teachers' (STs') lessons. We applied Fenstermacher's theory on practical arguments and found that MTs intervene primarily by guiding their pupils. MTs balance situational premises (e.g., ST and pupil characteristics, and triggers such as pupils behaving disruptively or STs making mistakes in the lesson content), value premises concerning mentoring and teaching, and empirical premises about the effects of intervening on STs' and pupils' well-being and development. We suggest MTs' intervening to not only cater to pupils' but also to STs' development needs.