This report on integrating mental health into primary care, which was developed jointly by the World Health Organization (WHO) and the World Organization of Family Doctors (Wonca), presents the justification and advantages of providing mental health services in primary care. At the same time, it provides advice on how to implement and scale-up primary care for mental health, and describes how a range of health systems have successfully undertaken this transformation.Mental disorders affect hundreds of millions of people and, if left untreated, create an enormous toll of suffering, disability and economic loss. Yet despite the potential to successfully treat mental disorders, only a small minority of those in need receive even the most basic treatment. Integrating mental health services into primary care is the most viable way of closing the treatment gap and ensuring that people get the mental health care they need. Primary care for mental health is affordable, and investments can bring important benefits.