Infidelity is one presenting problem that frequently arises in couple therapy. However, research-based treatment guidelines are lacking. Hoping to fill this gap, this chapter provides an overview of treating infidelity based on the intersystem approach first developed by Weeks. The chapter presents definitions and types of infidelity, etiology, assessment considerations, discernment counseling, a five-phase treatment approach, and directions for future research. The five-phase treatment approach includes crisis management and assessment, systemic considerations, facilitating forgiveness, treating factors that contributed to the infidelity, and enhancing intimacy through communication. Examples, questions clinicians can use in assessment sessions, cultural considerations, suggested interventions, and a culminating case study have all been included to aid therapists in their work with couples.