The very earliest years of life are key to fulfiled, productive and meaningful lives. Children's brain and physical development is at its most rapid during these first years as they develop skills and capabilities that affect lifetime outcomes as diverse as lifetime earnings, wellbeing and criminality. Gaps that open up between children, often along familiar lines of wealth and income, during this stage typically persist and are exacerbated over time. Thus, these years are key to understanding the transmission of poverty across generations. For many children growing up in poorer countries, these earliest years don't offer conditions that are always sufficient to reach their developmental potential. Poverty, malnutrition and disease-ridden and unstimulating environments can all contribute to children falling short developmentally of what they otherwise would have been capable of. Excitingly, however, a vibrant research agenda demonstrates that a child's development is not predetermined but highly malleable: it is heavily aected by environmental factors which can be altered by policy or behaviour change. This creates a clear rationale for intervening early in life, especially for the most disadvantaged children.