The present study adopted a praxis orientation to second language (L2) research and pedagogy to explore the professional development needs of three English as a foreign language (EFL) teachers after their first attempt at implementing dynamic assessment (DA) of essay writing. To date, few studies have examined how teachers learn to implement DA, especially in a context like Iran where the educational system has historically centered upon high-stakes assessments of learning (Rezagah, 2022). This study sought to understand how existing models of DA professional development could be modified to prepare teachers to implement DA of essay writing through written mediation. Findings revealed three challenges that arose for teachers during their initial attempts at DA implementation: (1) choosing an appropriate target for mediation, (2) delivering graduated mediation, and (3) dedicating sufficient time to the process of DA. We outline each of these three challenges in the findings of this article and their correspondent suggestions for revised future professional development of DA of writing.