The evolving nature of teachers’ teaching beliefs and the profound influence of those beliefs on teaching practice are widely recognized phenomena, and the sources of these beliefs continue to be documented. This study investigates pre-service teachers’ beliefs about language teaching and learning, the sources of their beliefs, and changes in these beliefs as determined upon their completion of a two-year TESL master’s program in Taiwan. The study’s sample consists of eight pre-service teachers. Analyzed following the grounded analysis method, the data includes the participants’ stated beliefs about English learning and teaching collected when they began the program and when they completed the program, classroom observations of their practice teaching, and interview responses both when they completed the program and several months after completion. The results show that the participants’ own English-learning experiences contributed significantly to their beliefs about language learning and teaching. By the end of the program confirmed by the participants’ self-reports of their teaching practices several months later, significant changes in their beliefs were found, particularly in relation to the use of communicative approaches in the classroom and the use of English as the medium of instruction in the TESL master’s program. The results of this study contribute to our understanding of how teacher’s teaching beliefs form and develop and the implications of these processes and the resulting beliefs for educating teachers.