Following the first 1,000 days of life that span from conception to two years of age, the ‘next 1,000 days’ of a child’s life from two to five years of age offer a window of opportunity to promote nurturing and caring environments, establish healthy behaviors, and build on early gains to sustain or improve trajectories of healthy development. This new two-part series Lancet on early childhood development focuses on the transition to the ‘next 1,000 days’ of the life course, describing why this developmental period matters, identifying the environments of care, risks, and protective factors that shape children’s development. The papers also estimate the number of children who receive adequate nurturing care, and examine whether current interventions are meeting children’s needs. In low- and middle-income countries (LMICs) 181·9 million children ages 3 and 4 are not receiving nurturing care, thus jeopardizing their development. The series summarizes the evidence, benefits and costs of ke