Infidelity can be one of the most traumatic experiences for couples in committed relationships. Of all the problems that lead couples to seek professional help, there are few more challenging than infidelity. In general, infidelity is a violation of the commitment to faithfulness within a devoted relationship (i.e., being married, cohabiting, or dating). However, infidelity can take various forms including emotional, physical, and Internet infidelity, and each may lead to deep feelings of betrayal, pain, anger, insecurity, or depression. This entry will define infidelity and further explore different types of infidelity, their prevalence and impact, cultural and contextual factors, and common approaches to treatment.