Coaching is proven to be an effective strategy of professional development; however, rare research has documented the nuances of coaching effects and the enablers in early childhood education. This study presents a systematic review of 33 randomised controlled trials or (quasi-)experiments to investigate the effects of coaching as a professional development tool on early childhood teachers' instruction and young children's developmental outcomes. The synthesis of evidence revealed positive effects of coaching on teachers' knowledge gains and instructional competencies and a range of children's developmental outcomes (language and literacy, social-emotional development, and academic skills). We analysed the ‘who’, ‘what’ and ‘how’ components of coaching to explain the enabling factors surrounding the effects of coaching. Empirical evidence synthesised in this review supports the theoretical notions of situated learning derived from anthropological studies of apprenticeship, suggesting that coaching should be situated in the real teaching context for promoting its effects. (author abstract)