Because relationship discord and dissolution are common and costly, interventions are needed to treat distressed couples and to prevent distress among vulnerable couples. We review meta-analytic evidence showing that 60-80% of distressed couples benefit from behavioral and emotion-focused approaches to couple therapy, but we also note that treatment effects are weaker in actual clinical practice than in controlled studies, dissipate following treatment for about half of all couples, and may be explained by factors that are common across models. Meta-analyses of prevention programs reveal reliable but smaller effects, reflecting a need to know more about whether and how communication mediates effects, about how risk and diversity moderate effects, and about how technology-enabled interventions can reduce attrition in vulnerable populations. Interventions for couples are improving and expanding, but critical questions remain about how and for whom they work. Expected final online publication date for the Annual Review of Clinical Psychology, Volume 16 is May 7, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.