--- - |2+ The goal of the current study was to examine the link between children's psychophysiology and aggression when both constructs were assessed simultaneously in scenarios designed to provide the opportunity to aggress for either a reactive reason or a proactive reason. Both sympathetic nervous system (SNS) activity (skin conductance) and parasympathetic nervous system (PNS) activity (respiratory sinus arrhythmia or RSA), as well as their interaction, were included as physiological measures. Participants were 35 5th‐grade children who were placed in two virtual‐peer scenarios; one scenario provided the opportunity to aggress in response to peer provocation (i.e., reactive aggression) and the other scenario provided the opportunity to aggress for instrumental gain (i.e., proactive aggression). Both skin conductance and RSA were assessed at the time that children were given the opportunity to aggress; this simultaneous assessment of psychophysiology and aggression allowed for an examination of in‐the‐moment relations between the two constructs. For the reactive scenario, RSA moderated the in‐the‐moment relation between skin conductance and aggression such that the association was positive at low RSA but negative at high RSA. For the proactive scenario, skin conductance negatively predicted aggression in‐the‐moment, and RSA positively predicted aggression in‐the‐moment, but their interaction was not a significant predictor of aggression. Theoretical implications for reactive and proactive aggression and underlying physiological processes are discussed. - Aggressive Behavior, Volume 44, Issue 6, Page 614-623, November/December 2018.