This study empirically derived a multiple risk factors model of the development of aggression among middle school students in urban, low-income neighborhoods, using Hierarchical Linear Modeling (HLM). Results indicated that aggression increased from sixth to eighth grade. Additionally, the influences of four risk domains (individual, family, community, and media) were tested individually and simultaneously, providing evidence that multiple risk variables at multiple levels influenced the development of aggression with varying degrees. While nearly all risk factors predicted variability in the initial level of aggression, few factors were adequate predictors of changes in aggression. Implications for future research and for evidence-based school interventions are discussed. (Contains 4 tables.)