This paper estimates the effects of junk food availability on body mass index (BMI), obesity, and related outcomes among a national sample of fifth graders. The researchers found that junk food availability does not significantly increase BMI or obesity among this fifth-grade cohort despite the increased likelihood of in-school junk food purchases. Estimates suggest that the caloric contribution of in-school junk food purchases are small, that the total amount of soda and fast food consumed in- and out-of-school are not significantly higher among children with greater availability of junk food in schools, and that there is little support for the idea that children substitute calories from healthy foods or increase their physical activity to compensate for increased junk food intake.