This paper presents a conceptual model which explains the challenges of providing high quality sustainable, teacher professional development at scale. It provides a framework to support holistic thinking at a systemic level, applicable across different systems. It draws on sociocultural theories of learning and encourages the user to think about the knowledge and skills required by actors at different levels of the system and the structures required to support their learning. It brings together the needs of teachers, school leaders and Education Officers (at the County, District or Provincial level). The empirical evidence for this model comes from a seven-year programme of activity supporting school-based professional development in Zambia. Evaluation findings highlight the importance of the role of mid-level professionals (District and Provincial Officers) in ensuring the sustainability of gains made through development projects. This is important because the professional development needs of those supporting teachers are often neglected.