Individual and dyadic stability models of newlyweds' physical aggression were evaluated in a sample of 394 newlywed community couples recruited at engagement and followed for 2.5 years. Aggression in young couples was hypothesized to be a stable, enduring trait, consistent with a latent state–trait conceptualization. However, the findings indicated that aggression can more parsimoniously be conceptualized as a “somewhat stable” trait with strong short-term correlations that gradually decrease at increasing intervals. Aggression was significantly dyadic. Men and women's aggression were consistently associated with one another across time beginning at engagement, with little evidence that one person's aggression evoked aggression in the partner in the time intervals studied. Consistency in a person's reporting of aggression, not shared with the partner, was strongest for self-reports. Aggr. Behav. 38:1-15, 2012. © 2011 Wiley Periodicals, Inc.