This factsheet from ARACY reviewed research and practice evidence in prevention and early intervention to identify factors that promote positive child development and how more effective systems can be developed to provide better life outcomes for children with a specific focus on when to intervene. The paper provides key insights for policy and practice, including: (1) Prevention and early intervention pays off as it is cheaper to fix current problems than to intervene in problems later.(2) Universal services help to identify needs early and prevent problems escalating. (3) The ‘best buys’ for thriving children are early investments in: the first 5 years, parenting and mental health. (4) There are other priority pathways for intervention with good evidence but investment in the 3 ‘best buy’ areas will have a substantial impact on a large number of factors for better outcomes for children.