Our once-in-a-lifetime opportunity.

Longitudinal studies of the determinants of children's health are complex, costly, infrequent, and incredibly valuable. It has become clear in recent years that the periconceptional environment plays a surprisingly large role in the health of the resulting child. This short introduction to this mini-monograph briefly recaps the articles included herein and reminds us that adequate forethought and planning will result in a study that could shed new light on the earliest determinants of children's health and thereby fill critical data gaps.