The first 1000 days of life is a critical period which sets the stage for long-term health. What are the implications of nutritional deficits during this period, and how can we apply this knowledge to middle- and low-income countries? Dr. Linda Adair begins with a conceptual framework that explains how maternal diet and micronutrient status affects genetic, epigenetic and cellular processes in the developing child. Parameters such as birth weight and postnatal growth play a key role in determining health outcomes in later childhood and adolescence. The COHORTS collaboration is an initiative that aims to understand how size at birth and growth in early life can influence adult outcomes, including the risk of obesity and related disorders. Adair presents key findings from the COHORTS study and places these in the context of other main trials conducted in high- and low-income countries. She concludes with a series of evidence-based recommendations with which we can optimize the health impact of the first 1000 days.