This longitudinal, 2-year study explored the experience of a cohort of seven in-service teachers in an English Language Teaching master’s program as they carried out action research studies to determine the impact of the stages of the research process on the various components of their pedagogical content knowledge. The research design followed a qualitative multiple case study model. Data were gathered through semi-structured interviews, written reflections, and participant observation journal notes. Main findings highlight the way specific components of the thesis linked to stages of the action research study contributed to developing certain types of knowledge integral to pedagogical content knowledge. Knowledge of the students was highly impacted by the needs analysis and the design and implementation of the intervention. In terms of pedagogical knowledge, building the state of the art and theoretical framework along with the intervention clarified and further developed knowledge of teaching methodologies and strategies. Subject matter knowledge was highly impacted by the theoretical framework and the socialization opportunities. Establishing the setting of the study contributed to further understand the constraints and affordances of their teaching contexts. Overall, this action research study became an insightful experience that helped participants promote effective classroom practices to address their students’ needs. Keywords: action research, longitudinal case study, master’s thesis, pedagogical content knowledge, second language teacher education.