A skilled workforce that understands the needs of children and can deliver high quality curricula to children with diverse needs, is required in order to realise the wide-ranging benefits that could follow from public investment in ECEC provision (such as in-service Professional Development (PD)). It is well established in the literature that high quality ECEC is important for children’s outcomes, and that variations in quality can be explained, to a large extent, by variations in educator pedagogy and practice. We argue for a better understanding of how ECEC educators and services can achieve (and be supported to optimise) children’s development and learning outcomes. This paper argues for more rigorous PD designs, considered evidence-based content and delivery that are embedded within a rich evidence-base of practice, and that focus on educator effectiveness, practice change, and also on the benefits to children’s learning and development. The paper draws upon effective PD literature and describes lessons learnt, focussing on the ‘who’, ‘what’ and ‘how’ of evaluation of effective in-service PD design and delivery.