Posttraumatic stress disorder (PTSD) was first included in the American Psychiatric Association's "Diagnostic and statistical manual of mental disorders" in 1980. Long used to describe the reactions of soldiers affected by stress in combat situations, PTSD is now recognised as a disorder affecting abused and neglected infants and children, survivors of assault and domestic violence, and disaster victims of all ages. How did a construct so distinctly associated with the experiences of adult combat veterans come to describe behaviours seen in children, and even infants? This transformation can be understood by examining the way clinical researchers have applied the "developmental sciences" (child and adolescent development, neuroscience, and cognitive science) to the study of trauma. Each of the developmental sciences has contributed to our understanding of children's reactions to traumatic situations, and each plays an integral part in the effort to understand PTSD as a disorder of children as well as adults.