Rapid internationalization of European higher education has resulted in a considerable increase in the number of English-medium instruction (EMI) degree programs now implemented at all levels of instruction. While this change of medium provides increased academic opportunities for all university stakeholders, the use of English by non-native speakers for teaching and learning in non-Anglosphere countries necessitates consideration of the ramifications of EMI. This study was motivated by the growing discussion of the challenges of English-medium instruction confronting lecturers for whom English is a foreign language. This case study investigated how 10 experienced lecturers in the natural sciences at the University of Copenhagen define their own teacher identity, and, their perceptions of any effects on their identity when shifting from Danish-medium instruction to English-medium instruction. This study utilized a multi-method approach to allow fuller access into the teachers' cognitions, and to overcome the weaknesses that arise from the use of self-report surveys to collect thoughts and perceptions. This approach comprised classroom observation of graduate level lectures, stimulated recall of these teaching events, and individual semi-structured interviews with the lecturers. The observations and stimulated recall served as a scaffold on which the interviews were built. In addition to questions directly focused on identity, the interviews also included two card sorting activities as elicitation devices. The analysis drew on the lecturers' comments and concerns related specifically to their underlying teacher cognitions about professional expertise, professional authority, and professional identity when teaching outside one's mother tongue in a multicultural, multilingual graduate setting. The results provide: 1) a model of teacher identity for lecturers in the natural sciences, 2) evidence that experienced NNS lecturers of natural science EMI do not find that the identified challenges of teaching in a foreign language affect their personal sense of teacher identify, and 3) reflections on teacher cognition studies. The lecturers highlight teaching experience and pedagogic content knowledge as factors that are at the core of their teacher identity. While the findings here report that these lecturers express confidence and security in the EMI context, the results also confirm the instructional and linguistic challenges identified in previous EMI research. This suggests that university managementneed to acknowledge these challenges, and develop and implement both linguistic and pedagogic competence training programs to support the needs of less experienced EMI lecturers.