The education and professional development of language teachers is a challenging endeavour – for (student) teachers as well as for the teacher educators involved. In particular, the meaningful integration of theory and practice is central to the ongoing discussion on how to conceptualise and implement foreign language teacher education. This article provides an overview of a research project, in which twelve (student) teachers enrolled in the M.A. programme 'E-LINGO – Teaching English to Young Learners' were investigated in a qualitative-interpretative case study. The study’s aim was exploring if, how and under what circumstances classroom action research, a core component of the programme, can foster professional development. Data from multiple sources were triangulated and analysed to elicit indicators for development in the form of critical learning incidents. The focus of this article will lie on affective factors, which, according to the results of this study, play a crucial role in teacher learning.