This chapter reports on a study investigating the role of Collaborative Learning Communities (CLCs) in the professional development of teachers who teach English as a Medium of Instruction (EMI) subjects, specifically in under-resourced contexts such as Vietnam. As EMI continues to become globally prevalent, driven by the increasing demand for English proficiency in higher education and in international working environments, it has become more important than ever to provide teachers with the necessary skills to effectively teach subject-area courses in EMI. Traditional professional development approaches, often characterized by top-down, one-size-fits-all training sessions, may not sufficiently address the specific challenges faced by EMI teachers. In contrast, innovative approaches including CLCs offer a more dynamic and participatory model of professional learning, grounded in collaboration, peer support, and reflective practice. This study, situated in the context of Vietnamese higher education, investigates the design, implementation, and impact of CLCs specifically developed to address the professional development needs of EMI lecturers in this under-resourced setting. The study employed a mixed-methods approach to analyze program impact, combining quantitative data from surveys with qualitative data from semi-structured interviews. Participants included nearly 100 EMI teachers of students majoring in English studies, translation-interpretation, management studies, and tourism and hospitality. The findings indicate that CLCs brought about a transformative and holistic professional development experience. Lecturers reported increased linguistic confidence, more use of student-centered teaching methods, and engagement in reflective practice. Participating in these communities also helped reduce feelings of isolation and fostered collegiality, especially among early-career lecturers. Cross-disciplinary collaboration in the CLCs enriched the range of pedagogical strategies shared and improved outlooks on EMI teaching. Participants emphasized the value of sharing resources, receiving feedback, and building professional relationships. Recommendations for future implementation include ensuring consistent and sustainable meeting schedules, expanding disciplinary diversity, and integrating research perspectives and activities such as action research and academic publishing on successful EMI teaching innovations. Ultimately, the assessment of program impact underscores the importance of teacher collaboration, reflection, and shared learning in advancing the competencies of EMI educators in ways that lead to engaged student learning in multilingual EI classrooms.