Objective: Intimate partner aggression (IPA) is a serious problem among dating couples. The present study examined dyadic and situational processes that may lead to IPA perpetration among a sample of 59 heterosexual couples (total n = 118), within the framework of Finkel’s I³ model. Method: IPA was assessed using an in vivo aggression task, in the context of a weak inhibiting factor (self-control depletion) and a strong impellance factor (negative emotion) generated during in vivo verbal conflict between partners (functioning as an instigating trigger). Results: Actor–partner interdependence model analyses demonstrated that negative emotion (prediscussion and reactivity) positively predicted men’s aggression and the interaction between emotion reactivity and self-control depletion predicted women’s partner aggression. Several partner effects emerged as well. Conclusion: These findings provide support for the I³ model and suggest that during conflictual encounters both partners may recognize and respond to each other’s negative mood and depletion states in ways that escalate aggression. The current study contributes to our understanding of the individual and dyadic processes leading to IPA perpetration. (PsycINFO Database Record (c) 2014 APA, all rights reserved)