The first 1000 days – the period from conception to the end of the child’s second year – is the period of greatest developmental plasticity, and what happens during this time can have life-long consequences for health and wellbeing. This presentation summarises the biological processes and environmental characteristics that shape development during the first 1000 days, and what impact these have over the life span. While the importance of the early years is now widely acknowledged, research in this area is rapidly advancing, and our understanding of the specific mechanisms that impact upon development is becoming more and more detailed and nuanced. This research has revealed whole aspects of biological functioning that were not previously recognised as playing a role in development. These include epigenetics, telomere effects, the role of the microbiome, and how all of these effects can be transmitted across generations. We have also learned about the broad environmental forces that shape these biological changes, including the developmental origins of health and disease hypothesis, social climate change, the mismatch hypothesis, and the social determinants of health and disease. This presentation describes what immediate experiences and exposures have this effect – including parenting experiences and family environments, physical environments and environmental toxins, nutrition, adverse experiences and stress, and poverty. The long-term impact of early experiences and exposures are described and implications for action explored.