A common belief is that talking about problems makes us feel better. In fact, seeking social support is related to well-being. However, if taken to a perseverative extreme, talking about problems can become problematic. The construct of "co-rumination" was developed to address this idea. Co-rumination refers to talking excessively about problems and is characterized by rehashing problems, speculating about problems, and dwelling on negative feelings. Co-rumination is typically studied in children's and adolescents' friendships and has adjustment trade-offs. Like rumination, co-rumination is associated with internalizing symptoms. However, co-rumination also is associated with having high-quality relationships, presumably due to social sharing. In this article, I review what it means to co-ruminate; how co-rumination is assessed; the roles of age, gender, and relationship context; associations with adjustment; and the role of interpersonal stress and problems. I close by considering directions for research and applied implications.