This study aimed to identify teachers' perceived challenges in implementing assessment-as-learning (AaL) in classrooms, and evaluate the impact of a one-year professional development programme (PDP) designed for 47 teachers in a Hong Kong primary school to promote AaL implementation. Findings showed that the PDP increased the frequency and appropriateness of AaL implementation when on-site support was also provided, otherwise changes were not substantial. It suggests that systematic supporting measures should be provided together with PDPs to address the challenges teachers encountered in implementing AaL.