English-medium instruction (EMI) has become a driving force behind the curriculum reform in higher education around the world. The current study, drawn from a larger project that involves an EMI teacher development program, explores subtle nuances of change in teachers' self-efficacy beliefs, employing Bandura's theory of self-efficacy (Bandura, 1977, 1986) as a framework, among university teachers who have completed the training program. A total of 11 faculty members from five universities in Northern Taiwan were individually interviewed, along with data from their EMI classroom observations. Results indicated that novice EMI teachers may experience a weakening of self-efficacy that could hurt their commitment to teaching. After completing the EMI teacher development program, participants reported gaining more confidence in using the teaching techniques taught in the program. Most importantly, the program helped them re-examine their role as an EMI instructor and sensitized them to the students' perspective that they had not been aware of before. The new insights humbled them, yet empowered them for their EMI undertaking. The transformation of teachers' self-efficacy allowed these EMI teachers, juniors or seniors alike, to continue to grow as they struggled through teaching in a foreign language. Implications of the findings and suggestions for future EMI teacher development programs are also provided.