Recent increases in the prevalence of childhood obesity have created an urgent need for preventive strategies, but such strategies in turn depend on an improved understanding of the etiology of pediatric obesity. There is a dearth of evidence of the cause of pediatric obesity at present, with much of the literature of limited quality, inconclusive, and contradictory. The present review highlights the paradox of energy imbalance-its apparent simplicity but actual complexity-and the difficulties in etiologic research that arise from this complexity. The review identifies a number of emerging problems for etiologic studies. The review also makes a number of proposals that might improve future etiologic studies and provides a framework for integrating the diverse body of evidence of etiology that will become available in future. Gathering improved evidence of etiology, and then integrating and interpreting it, will take many years. In the meantime, an emphasis on developing more effective preventive interventions is necessary.