When children are sick, clinicians can consult a known standard for how their heart, lungs and kidneys should be growing and working. But no such rubric exists for something as complex as brain development. As part of a nationwide consortium, the HEALthy Brain and Child Development Study (HBCD) is set to provide a comprehensive picture of healthy development. This longitudinal study of infants and children, unprecedented in scope and in aim, will provide information that will impact medical care for decades to come.