While research on teachers' Critical Incidents (CIs) has grown exponentially, little research has been done on teachers' assessment-related CIs, much less examining the contributions of such CIs to their identity sense-making. This study aimed to fill some of this gap by exploring assessment-related CIs of 13 Iranian L2 teachers and the associated contributions to their identity development. Data were collected from narrative frames and cognitive interviews and analyzed via grounded theory and thematic analysis. The results indicated that the teachers narrated more teaching low incidents than teaching high incidents. The CIs had influenced the teachers' self-images, had mediated the way they defined their role in relation to other institutional participants, and were accompanied by institutional restrictions that had limited the teachers in enacting their preferred assessment practices and developing their autonomy. Results of the study offer implications to policy-makers and teacher educators regarding the importance of unpacking teachers' assessment CIs to enhance the effectiveness o f their educational systems.