A picture of effective professional development for science teachers is emerging, with a number of educational jurisdictions now drawing on and requiring that professional development is designed in ways that are informed by evidence. However, professional development occurs in vastly different educational ecosystems, with outcomes that are varied and sometimes difficult to measure, especially for disciplinary studies at the primary school level. In this chapter, the first author (SC) draws on their experiences working in primary schools providing professional development in STEM to primary teachers, with the second author (HG) providing critical reflections. This chapter will be centred on effective professional development principles, enriched with case studies that more fully describe the evolving nature of professional development dictated by a range of factors, including teacher, student and systemic needs.