The ‘First Thousand Days’ refers to the period from conception to the child’s second birthday. It is increasingly gaining traction as a concept to guide public health policy. It is seen as a crucial window of opportunity for interventions that improve child and population health. This review outlines the origin and growth of the First Thousand Days concept, and the evidence behind it, particularly in the areas of brain development and cognition; mental and emotional health; nutrition and obesity; programming and economic benefits. The review then describes UK experience of use of the concept to inform policy, and a recent government inquiry that mandates more widespread implementation.