This article, a follow-up on our methodological review of infidelity studies, provides a substantive review of the research findings on infidelity in committed relationships. The aim of this article is to present the most conclusive findings available to both researcher and practitioner on the subject of infidelity. We highlight attitudes toward infidelity; prevalence data; types of infidelity; gender dynamics and infidelity; issues in the primary relationship and their relationship to infidelity; race, culture, and infidelity; education, income, employment, and infidelity; justifications for infidelity; individual issues and their relationship to infidelity; same-sex couples and infidelity; attachment and infidelity; opportunity and infidelity; the aftermath and recovery process from infidelity; and clinical practices.