There is a large literature on the possible association between obesity and asthma or asthma-like symptoms. However, no previous review has been confined to studies in children and adolescents. Studies vary in definitions of overweight or obesity and in whether outcomes were parent-reported symptoms or doctor-diagnosed asthma; there is no consistency in the findings. Three studies provide evidence for a greater incidence of diagnosed asthma in those obese at baseline compared to those not obese. The evidence for a stronger association in girls than in boys is weak. No study in a Western society has found objective evidence for the association, and results might be due to increased reporting of symptoms in obese children or to diagnostic bias. Intervention studies to prevent or reduce obesity could provide data to support or refute the association.