Most people in Euro‐American and industrialized societies strongly believe in fidelity and most are, in fact, sexually faithful to their romantic partners. However, infidelity (also called extramarital or extrarelational sex) does occur. Several factors increase the likelihood of infidelity (e.g., gender, frequent travel); of these, relational factors (e.g., conflict, dysfunctional communication) are the most powerful. Infidelity has many negative consequences. The cheating partner often experiences guilt, anxiety, and fear; the cheated‐on partner typically experiences feelings of jealousy, anger, self‐doubt, and betrayal; and the relationship itself often does not survive. Researchers who study responses to (imagined) infidelity generally find that both men and women consider emotional infidelity (their partner falling in love with someone else) to be more upsetting than sexual infidelity (their partner engaging in sexual activity with someone else). However, two sex differences are commonly found: when asked to select the most distressing event, more men than women select sexual infidelity, and more women than men choose emotional infidelity. Although scholars continue to debate the origin of these sex differences, all agree that infidelity is an upsetting and destructive event in the life of most monogamous relationships.