<p>STEM education is widely recognised as beneficial in enabling learners to develop the knowledge, skills and dispositions needed to navigate the challenges and opportunities of the modern world. Primary educators are fundamental in supporting the STEM learning of their students through appropriate continuing professional development (CPD), which provides teachers with ongoing professional learning activities to support improvements in knowledge, skills, confidence and understanding relating to teaching. While post-primary STEM contexts receive considerable scholarly attention, primary STEM teacher CPD remains under-explored by comparison. This paper shares empirical insights from a study which explored the influence of a CPD programme on Northern Irish primary teachers’ (<i>n</i> = 22) self-efficacy in STEM. Analysis of data from surveys, focus group interviews and participants’ reflective diaries indicated that the programme enhanced teachers’ overall self-efficacy and facilitated development of their STEM knowledge. Participants also identified prevailing issues which constrained their capacity to facilitate effective STEM learning experiences for their students. These included resourcing, along with wider issues of policy, curriculum and assessment priorities in the Northern Irish education context. For practitioners, researchers and policymakers, this paper offers fresh evidence and original insights into how teachers can be supported in STEM teaching and learning through effective CPD. We contend that teachers and schools must be supported appropriately through leadership, policy and resourcing at all levels if the educational, economic and social benefits of STEM for learners is to be realised in practice.</p>