This longitudinal study of adolescent girls and boys examined the contributions of social (peer appearance context), psychological (internalized appearance ideals and appearance social comparison), and biological (body mass) factors to the development of body dissatisfaction. Students (165 girls and 139 boys) completed questionnaires when they were either in 7th grade or 10th grade and again 1 year later. The results for the boys revealed a singular pathway to body dissatisfaction through internalized commitment to muscularity ideals. The prospective analyses of change in body dissatisfaction among the girls reflected the contributions of appearance conversations with friends, appearance social comparisons, and body mass. There was no evidence of mediation among the boys and limited support for it among the girls.