This working paper from the National Scientific Council on the Developing Child defines the concept of “toxic stress”—what happens when children experience severe, prolonged adversity without adult support. It discusses how significant adversity early in life can alter a child’s capacity to learn and adapt to stressful situations, as well as how sensitive and responsive caregiving can buffer the effects of such stress. The paper also suggests how to create policies that minimise the disruptive impacts of toxic stress on young children.