This longitudinal case study examined the impact of in-service teacher professional development (PD) on teacher cognition relative to multilingualism and pedagogical practices in linguistically diverse classrooms. Two teachers of English as an additional language (EAL) at a primary school in Norway, who teach large numbers of linguistically and culturally diverse students, participated in twelve workshops that focused on both theoretical and pedagogical aspects of language learning and multilingualism. Drawing on questionnaire and classroom observation data collected during two phases of the project (before and after PD), the findings suggest that the teachers displayed individual trajectories in the development of teacher cognition and practices. While one of the teachers showed considerable change over time, the other one remained more stable, suggesting that individual differences, such as language and family background, education, and teaching experience mediate the impact of PD. Implications for local PD at the site of data collection and broader implications for PD in multilingual contexts are discussed.