This paper attempts to identify the leadership practices and qualities of school principals engaged in implementing a curriculum reform in Hong Kong. Based on interview data of school principals, this paper shows that the types of instructional leadership practices that school principals adopt and the contextual conditions in which these practices occur are key factors explaining the differential effectiveness of implementation efforts. Six efficacious instructional leadership practices and two contextual conditions critical to their development were identified. This paper reconceptualises instructional leadership as a collective and transformational endeavour, functioning in communities of practice to support school development at different levels.