The PedsQL (Pediatric Quality of Life Inventory) (Children's Hospital and Health Center, San Diego, California) is a modular instrument for measuring health-related quality of life (HRQOL) in children and adolescents ages 2 to 18. The PedsQL 4.0 Generic Core Scales are multidimensional child self-report and parent proxy-report scales developed as the generic core measure to be integrated with the PedsQL Disease-Specific Modules. The PedsQL 4.0 Generic Core Scales consist of 23 items applicable for healthy school and community populations, as well as pediatric populations with acute and chronic health conditions. The 4 PedsQL 4.0 Generic Core Scales (Physical, Emotional, Social, School) were administered to 963 children and 1,629 parents (1,677 subjects accrued overall) recruited from pediatric health care settings. Item-level and scale-level measurement properties were computed. Internal consistency reliability for the Total Scale Score (alpha = 0.88 child, 0.90 parent report), Physical Health Summary Score (alpha = 0.80 child, 0.88 parent), and Psychosocial Health Summary Score (alpha = 0.83 child, 0.86 parent) were acceptable for group comparisons. Validity was demonstrated using the known-groups method, correlations with indicators of morbidity and illness burden, and factor analysis. The PedsQL distinguished between healthy children and pediatric patients with acute or chronic health conditions, was related to indicators of morbidity and illness burden, and displayed a factor-derived solution largely consistent with the a priori conceptually-derived scales. The results demonstrate the reliability and validity of the PedsQL 4.0 Generic Core Scales. The PedsQL 4.0 Generic Core Scales may be applicable in clinical trials, research, clinical practice, school health settings, and community populations.