The causes of the current high prevalence of overweight and obesity among children are not clearly known. Schools have been implicated in the causal chain to high child obesity prevalence. Recent studies have compared school year versus summertime changes (herein called seasonal differences) in child adiposity or related phenomena. The most common seasonal pattern in six longitudinal descriptive studies was that overweight and obese children experienced accelerated gain in weight or some BMI indicator during the summer, whereas healthy weight children gained less or not at all. Four physical activity (PA) intervention studies demonstrated that school year fitness improvements were lost during the summer. One study showed that PA declined across the summer. Another study provided conflicting results of lower total energy expenditure in the summer, but no seasonal difference in total energy expenditure after adjusting for fat-free mass. This pattern of fairly rapid seasonal differences suggests that PA is the primary factor contributing to seasonal differences in weight or BMI, but the documented seasonal pattern in PA (i.e., higher in summer) does not support this relationship. Sleep duration has also been inversely related to child adiposity. Seasonal patterns in adiposity, PA, and sleep need to be clearly established separately for overweight and healthy weight children in further longitudinal research to provide a clear focus for national policy.