This report presents the findings of a SCoTENS funded study with practicing language teachers in Ireland, within the framework of Threshold Concepts (Meyer, Land et al. 2010), to identify the core but troublesome knowledge and practices of language teaching and the conditions that facilitate the integration of these concepts over a teaching career. The project draws on the voice of professionals in the two jurisdictions, North and South, articulating key points of learning in their careers. Qualitative analysis of participant interviews suggests that, while the threshold concepts identified are quite consistent across both jurisdictions, their articulation and implementation are contingent upon the local and broader policy context in which teachers work. The concepts are expressed as a belief system where contextual factors mediate the degree to which teachers can teach in accordance with their beliefs.