: Long-term committed mating is a fundamental strategy in the human repertoire. Successful enactment of this strategy requires solving two related adaptive problems--fending off potential mate poachers and preventing a mates from defecting. Mate guarding adaptations evolved to solve these persistent problems. Those who failed in mate guarding risked suffering substantial reproductive costs ranging from genetic cuckoldry to reputational damage to the entire loss of a mate. Because the precise nature of the adaptive problems confronted differed historically for the sexes, men and women evolved corresponding differences in the underlying psychology of mate guarding. Men's mate guarding, relative to that of women's, is strongly triggered as a consequence of being mated to young and physically attractive women, being confronted by interested rivals who have superior economic resources or prospects, and having a mate who displays signs of sexual involvement with a rival. Women's mate guarding, relative to that of men's, is triggered as a consequence of being mated to men high in income and status striving, rivals who are more physically attractive, and having a partner who shows signs of emotional involvement with another woman. Behavioral output of mate guarding adaptations range from vigilance to violence.