In the past few decades, increasing demands have been placed on developing students' autonomy and critical thinking skills. To enhance students' agency in learning, teachers have been encouraged to adopt alternative pedagogical practices. The revamp of pedagogy needs to be reflected in assessment as it is an integral part of instruction. Practising teachers, however, lack assessment knowledge due to inadequate training during the pre-service preparation programmes. The enhancement of assessment knowledge is being urgently called for, which demands the professional development of teachers. This study seeks to investigate the impact of a professional development programme organized by a university in Hong Kong on the development of assessment knowledge of four teachers in two elementary schools. Findings indicate that the acquisition of assessment knowledge is a long-term achievement, which requires ongoing professional development efforts. Copyright © 2019 Informa UK Limited, trading as Taylor & Francis Group.