The review uncovered numerous research gaps. In particular, more research is needed to examine whether effective treatment of depression in primary care can alter the course of chronic disease. Such studies will require larger samples, longer time frames, and joint funding from multiple institutes. In addition, many real-world primary care patients have multiple comorbidities, and “perhaps what the field needs most to understand,” the report notes, “is what models of care work best for patients with common clusters of disease.” Diabetes, hypertension, and obesity, concomitant with depression, is such a cluster. More studies are also needed of collaborative care for primary care patients with other mental disorders, such as anxiety, psychotic disorders, substance use disorders, and cognitive disorders. In addition, the literature review found no studies of colocation of mental health services and no head-to-head trials of various approaches that would enable researchers to identify the active ingredients of specific models.