Chronically critically ill pediatric patients represent an emerging population in NICUs and PICUs. Chronic critical illness has been recognized and defined in the adult population, but the same attention has not been systematically applied to pediatrics. This article reviews what is currently known about pediatric chronic critical illness, highlighting the unique aspects of chronic critical illness in infants and children, including specific considerations of prognosis, outcomes, and decision-making. We propose a definition that incorporates NICU versus PICU stays, recurrent ICU admissions, dependence on life-sustaining technology, multiorgan dysfunction, underlying medical complexity, and the developmental implications of congenital versus acquired conditions. We propose a research agenda, highlighting existing knowledge gaps and targeting areas of improvement in clinical care, research, and policy.