The development of bullying behavior was examined across three years in a sample of 515 adolescents (46% females) from 41 classrooms. At time 1, the students were in grades 9 and 10 (mean age = 14.5 years; SD = 0.54). Results of a multilevel growth model showed that both baseline level and change of bullying varied significantly across individuals as well as across classrooms. At the individual level, gender, aggression and competition for social dominance were related with baseline level of bullying. Competition for social dominance and class change were additionally associated with increases in bullying over time. At the classroom level, pro-bullying behaviors were associated with higher baseline level of bullying, whereas anti-bullying behaviors with decreases in bullying over time. Finally, a cross-level interaction underlined that the link between aggression and bullying was moderated by the pro-bullying behaviors within each class. Results are discussed according to the child by environment perspective. (Contains 2 tables and 3 figures.)