The science is clear: A child's brain is built, not born. The process begins before birth and involves a complex interplay of neural connections that are shaped by experience and environment. In the early years, these neural connections occur at lightning speed--a speed never again repeated. They establish a foundation of development that will help children grow, learn and thrive. This process is fuelled by adequate nutrition, protection from harm and responsive stimulation including early learning opportunities. Unfortunately, many millions of children around the world are deprived of the ingredients that foster optimal brain development. They do not have nutritious food or health care; they are not protected from violence, extreme stress, pollution and conflict; they are starved of responsive stimulation from a caring adult; and they miss out on opportunities to learn. Negative experiences can slow down and alter how neural connections are made in a baby's brain. They can impact a child's ability to grow and learn, and they can even reduce earnings as an adult by up to 25 per cent. This report presents data and outlines best practices and policies that can put governments on the path to providing every child with the best start in life. It outlines the neuroscience of early childhood development (ECD), including the importance of nutrition, protection and stimulation in the early years. And it makes the case for scaling up investment, evaluation and monitoring in ECD programmes. The report concludes with a six-point call to action for governments and their partners to help maximize the potential of the children who will build the future--by making the most of the unparalleled opportunities offered by the early moments in life. [Support provided by the H&M Foundation.]