Aggression is an affect-laden behavior. The within-person variability of affective states that immediately precede, accompany, and follow aggression-and their links to between-person variability in aggressive behavior and traits-remain incompletely understood. To address this gap in our understanding, we examined 8 studies in which 2,173 participants reported the negative and positive affect they experienced before, during, and after a laboratory or online aggression task. We quantified the within-person variability within and across negative and positive affect intensity, as well as the variability in oscillations between negative and positive affect Internal meta-analyses revealed an association between aggressive behavior and traits and flux in positive affect (against our preregistered predictions). Probing this effect with piecewise growth models showed that less aggressive individuals exhibited a pronounced decrease in positive affect during aggression, as compared to before and after the act. This downward fluctuation in positive affect was attenuated among aggressive individuals, who exhibited relatively stable levels of positive aggression-related affect. Thus, stable positive affect surrounding an aggressive act and higher positive affect during the act may buttress and promote aggressive tendencies. These findings support a reinforcement model of aggressive behavior, contrast with the aggression literature's conventional focus on negative affect and the instability thereof, and point to the utility of dynamic measures of moment-to-moment affect in understanding human social behavior. (PsycInfo Database Record (c) 2021 APA, all rights reserved).