To review existing definitions of prolonged mechanical ventilation in children. Review of the literature. Not applicable. Pediatric patients under mechanical ventilation. Not applicable. We searched the literature for original research on prolonged mechanical ventilation in children using the PubMed, Embase, and Cochrane Library databases. Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed, and we extracted patient characteristics, duration of mechanical ventilation, outcomes, and associated complications. We identified 30 articles for qualitative analysis and found a wide variety of definitions, varying from 48 hours to 6 months. The most frequently used definitions of prolonged mechanical ventilation were greater than 7 days, greater than 14 days, and greater than 21 days. There was no high-quality evidence of a definition for prolonged mechanical ventilation in children. There is limited evidence to determine the definition of prolonged mechanical ventilation in children. The variety of definitions is a sign of the need for a consensus definition of pediatric prolonged mechanical ventilation.