Exploring Iranian ESP teachers’ language-related critical incidents

English for specific purposes (ESP) practitioners who are content experts experience different types of critical incident (CI). Although CIs can influence the success or failure of ESP courses and impact on ESP practitioners’ professional lives significantly, they have received only scanty attention in the ESP literature. This study investigates language-related CIs experienced by ESP instructors who are content experts. Twenty-seven CIs were identified via narrative frames (n = 17) and interviews with Iranian ESP instructors (n = 10). Of them, the ten language-related CIs were analyzed in terms of their nature, the strategies and tactics that the ESP practitioners utilized to tackle them, and the lessons that they learned from them. The language-related CIs centered on pronunciation difficulty, weak grammar, insufficient skills in teaching reading and writing, low competence in language testing, and unfamiliarity with research on academic genres. In their response to the CIs, the ESP instructors deployed three types of coping strategy: admitting ignorance, avoidance, and risk-taking. They utilized different tactics to manage their CIs and reported different lessons learned. These findings have important implications for ESP teachers’ professional development and ESP teacher education.