English language teachers are expected to use their talk, "teacher talk," effectively; however, teacher education courses do not always clarify how student teachers can achieve this. This article advocates that understanding and using effective teacher talk is crucial for successful ELT and as such is a "threshold concept" in TESOL. "Threshold concept" is a term used in higher education to describe core concepts that, once understood, transform perception of a subject and help students progress in their discipline. In this study, nine native speaker and non-native speaker EFL student teachers explore their use of teacher talk by participating in stimulated recall interviews using recordings of teaching practice lessons as the stimuli. Findings show trainees understand effective teacher talk differently, with some trainees linking it to pedagogical purpose whilst others do not. This study is the first to examine threshold concepts in TESOL and shows it is a useful micro-perspective for developing the TESOL teacher education curricula.