The purpose of this research was to examine the implementation of a professional learning project aimed at building educators' knowledge and skills in assessment for learning (AfL) within two school districts in Ontario, Canada. Specifically, the research examined the value of a two-tier "Instructional Rounds" (IR) professional learning model. This professional learning model was unique because it engaged both teachers and principals in collaboratively learning and implementing AfL strategies in order to develop systemic capacity in assessment. In total, 12 principals, 48 teachers, two superintendents and two school district assessment consultants participated in the study. Data were collected through observations of IR sessions, classroom observations, interviews, IR session reflections and a post-project survey. Findings from this study report on positive changes in teachers' and principals' conceptions and implementation of AfL as well as on the value and challenges of IR as a professional learning model. The paper concludes with a discussion on developing systemic capacity in AfL through an IR model of professional learning.