This article attempts to unpack the complexity of teachers' professional knowledge construction in Assessment for Learning (hereafter, AfL). It presents a qualitative study of a school-based AfL Project which took place in a secondary school in Hong Kong. Thirty lessons video-recorded in the AfL Project and nine teacher interviews conducted after the Project were analyzed. The findings enrich our understanding of professional knowledge construction in four aspects: (1) personal nature of teachers' practical knowledge construction in contrived situations; (2) confined integration of AfL into pedagogical content knowledge in terms of "skill progression that accompanies experience"; (3) more sophisticated integration of AfL into pedagogical content knowledge as "practicalizing theoretical knowledge"; and (4) reflection, conscious deliberation, and "theorizing practical knowledge" associated with more sophisticated professional knowledge integration. Implications for enhancing teachers' professional learning are discussed.