Mental health has always been a low priority worldwide. Yet more than 650 million people are estimated to meet diagnostic criteria for common mental disorders such as depression and anxiety, with almost three-quarters of that burden in low- and middle-income countries. Nowhere in the world does mental health enjoy parity with physical health. Notwithstanding astonishing medical advancements in treatments for physical illnesses, mental disorder continues to have a startlingly high mortality rate. However, despite its widespread neglect, there is now an emerging international imperative to improve global mental health and wellbeing. The UN's current international development agenda finalised at the end of 2015 contains 17 Sustainable Development Goals (SDGs), including SDG3, which seeks to ensure healthy lives and promote wellbeing for all at all ages. Although much broader in focus than the previous eight Millennium Development Goals (MDGs), the need for worldwide improvement in mental health has finally been recognised. This Handbook addresses the new UN agenda in the context of mental health and sustainable development, examining its implications for national and international policy-makers, decision-makers, researchers and funding agencies. Conceptual, evidence-based and practical discussions crossing a range of disciplines are presented from the world's leading mental health experts. Together, they explore why a commitment to investing in mental health for the fulfilment of SDG3 ought to be an absolute global priority.