The goal of this paper is to introduce an emerging theoretical lens and strand of research in the field of language teacher education, that of threshold concepts (TCs). We do so by a) defining and explaining the construct of TCs itself, b) systematically reviewing the small body of literature that has explicitly used a TC lens to research language teacher education and c) discussing the implications of using this construct for research and practice. To find and select papers, we used a combination of inductive and deductive searches with Scopus, Google Scholar, and Research Gate. Papers not using TCs as an explicit theoretical lens were excluded. The review of the literature shows that 5 threshold concepts have been identified so far in language teacher education: interlanguage, effective teacher talk, real language use, fostering learner autonomy, and grammatical rank scale. All but one of the threshold concept studies in language teacher education have been conducted in the British Isles. The only non-British paper is by a Mexican author and was conducted and published in Mexico. Action research has been the preferred design. Other established methods for identifying threshold concepts have not been applied to language teacher education, which means that there is much potential for expanding and improving upon existing research. We suggest that threshold concepts hold potential to both investigate and enhance the disciplinary identity of language teachers.