Despite a tendency to form socially monogamous pair-bonds that carry expectations of sexual exclusivity, infidelity has been a recurrent feature of human mating across societies. The attitudes, social cognition, affect, and behavior associated with infidelity vary in patterned ways between women and men. In the current chapter, we use an evolutionary perspective to make sense of the historical and cross-cultural ubiquity of extradyadic behavior, as well the adaptative costs and benefits of men’s infidelity. Specifically, we review theory and research pertaining to men’s extra-pair mating and consider salient individual differences, romantic relationship dynamics, and social–ecological factors that influence mating strategies and extradyadic involvement. Following other scholars, we argue that men have evolved adaptations for short-term mating that facilitate opportunistic extra-pair behavior in a “quantity-over-quality” reproductive strategy. Consequently, on average, men are predicted to express a stronger desire to engage in sexual infidelity and to have more permissive attitudes toward extradyadic involvement than women. However, only particular men appear to execute a mixed mating strategy involving a long-term mate and an extra-pair partner, such as those with greater mate value. Satisfaction with and commitment to the relationship appear to be crucial in preventing men’s infidelity, and socio-ecological factors, including cultural dynamics (e.g., norms surrounding infidelity) and sex ratios that create conditions of mate scarcity, are inextricably tied to men’s extra-pair mating.