Psychological research has produced a rich body of empirical data documenting humanity's propensity to commit infidelity in the context of long-term relationships, but comparatively little work has been dedicated to synthesizing these data into an integrated framework that encompasses the full range of its processes, from why it occurs in the first place to how it affects the long-term relationship thereafter. This edited handbook integrates a broad range of topics such as characteristics related to the propensity to commit infidelity, sex differences in reactions to infidelity, our inclination to dissolve relationships after infidelity, and other responses to infidelity. It showcases contributions from experts in social psychology, evolutionary psychology, and others who specialize in research on romantic relationships. The handbook discusses the processes of infidelity alongside sources of variation, such as sexual orientation, developmental life history, individual differences, and culture. This volume captures the interdisciplinary quality of research on the predictors, nature, and consequences of infidelity for the broader social scientific community interested in trust in romantic relationships.