Despite the urgent public health implications, relatively little is yet known about the effect of peers on adolescent weight gain. We describe trends and features of adolescent BMI in a nationally representative dataset and document correlations in weight gain among peers. We find strong correlations between own body mass index (BMI) and peers' BMI's. Though the correlations are especially strong in the upper ends of the BMI distribution, the relationship is smooth and holds over almost the entire range of adolescent BMI. Furthermore, the results are robust to the inclusion of school fixed effects and basic controls for other confounding factors such as race, sex, and age. Some recent research in this area considers whether or not adolescent weight gain is caused by peers. We discuss the econometric issues in plausibly estimating such effects while accounting for growth spurts and difficulties in defining adolescent obesity. While our work identifies correlations between adolescent BMI and peers' BMI, it is not intended to and cannot fully address the existence of endogenous peer effects.