Committed, long-term romantic relationships are ubiquitous among modern society. They are one of the most important contexts for the development, maintenance, and treatment of psychopathology. In this review, I first place psychopathology within the most commonly cited theoretical model of marital satisfaction and stability and then discuss how relationship satisfaction is conceptualized and assessed in this literature. In the second half of the review, I describe the theories regarding how romantic relationships may be connected to psychopathology. Relationship distress is easily incorporated into a diathesis-stress model as an important trigger for psychopathology. Next, I review cross-sectional research, longitudinal research, and treatment efficacy research linking relationship quality and psychopathology. I provide evidence for the robustness of these effects and areas where research must expand. I finish with a summary section that synthesizes what is known about the mechanisms linking relationship distress and psychopathology. Expected final online publication date for the Annual Review of Clinical Psychology, Volume 17 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.