Interpersonal Psychotherapy (IPT) is an affect- and relationally focused, time-limited treatment supported by research spanning >4 decades. IPT focuses on stressful interpersonal experiences of loss, life changes, disputes, and social isolation. It emphasizes the role of relationships in recovery. This scoping review describes, within a historical perspective, IPT's evolution as an evidence-supported treatment of psychiatric disorders. English-language publications (n = 1119) identified via EMBASE, MEDLINE, PsycINFO, and Web of Science databases (1974-2017), augmented with manual reference searches, were coded for clinical focus, population demographics, format, setting, publication type, and research type. Quantitative and qualitative analyses identified IPT publications' characteristics and trends over four epochs of psychotherapy research. IPT literature primarily focused on depression (n = 772 articles; 69%), eating disorders (n = 135; 12%), anxiety disorders (n = 68; 6%), and bipolar disorder (n = 44; 4%), with rising publication rates and numbers of well-conducted randomized, controlled trials over time, justifying inclusion in consensus treatment guidelines. Research trends shifted from efficacy trials to effectiveness studies and population-based dissemination initiatives. Process research examined correlates of improvement and efficacy moderators. Innovations included global initiatives, prevention trials, and digital, web-based training and treatment. Sparked by clinical innovations and scientific advances, IPT has evolved as an effective treatment of psychiatric disorders across the lifespan for diverse patients, including underserved clinical populations. Future research to elucidate mechanisms of change, improve access, and adapt to changing frameworks of psychopathology and treatment planning is needed. IPT addresses the universal centrality of relationships to mental health, which is as relevant today as it was over 40 years ago.