Intimate partner violence (IPV) is a paradoxical combination of affection and aggression. So why do people show an all-too-frequent tendency to harm their loved ones? Towards answering this question, we review a broad literature that explicates the ultimate and proximate roots of IPV perpetration. At the ultimate level, IPV perpetration is likely to be the result of evolutionary and socio-structural forces. Theories of aggression are then brought to bear in order to articulate the proximal sequence of psychological processes that magnify and constrain IPV. Interpersonal (e.g., rejection), intrapersonal (e.g., self-regulation), and biological (e.g., testosterone) factors are discussed in their relation to IPV perpetration. Finally, potentially fruitful avenues for intervention are evaluated, as exemplars of the hope that a robust understanding IPV perpetration will lead to the reduction of this costly behavior.