Identifying Depression Early in Adolescence (IDEA) King's-Brazil-Nepal-Nigeria network Depression is a common mental disorder that will affect up to one out of five people around the world during their lifetimes and tends to start in adolescence. Depression is an especially debilitating mental illness because it often begins early in life and has a chronic course with many depressive episodes occurring throughout one's lifetime. Many people lack access to depression care and this is especially true in the world's poorest countries. Our proposed collaboration aims to bring a high-income country's expertise in research, where most of the scientific knowledge about depression and other mental health problems has been produced, to developing countries, where 9 out of 10 children and adolescents live. This collaboration between King's College London in the UK and researchers from Brazil, Nepal and Nigeria intends to find new ways to identify which adolescents are at risk of depression, what protects young people from developing depression, and are these factors the same or different in these contrasting contexts. Such information is crucial to help us understand how to intervene early in a range of settings to prevent a lifetime of suffering.   In order to achieve this, we propose to do the following: First, we will bring together researchers with different types of expertise from high (UK), middle (Brazil and Nigeria) and low (Nepal) income countries to reach a consensus about how to measure risk and protective factors for depression in comparable ways among adolescents in these different countries. We will then draw upon existing datasets of youth who have participated in research at multiple time points during their childhood and adolescence to run initial tests to see whether similar or different factors predict the development of depression in each country. This collaborative network will then apply for funding to conduct a new research project to assess much larger numbers of adolescents in each country with the consensus measures to identify the biological, social, psychological, and environmental factors that predict which adolescents will develop depression and which will not in each of these settings. Finally, we will scope out potential gaps in equipment, expertise, and policies in each country so that we know what needs to be put in place before such a large-scale research project can be successfully conducted. In order to begin the process of rectifying these gaps, we will deliver online training and offer hands-on apprenticeships to increase the skills of researchers in Brazil, Nepal and Nigeria to enable them to effectively conduct the research themselves.  Ultimately, this research will generate new knowledge on how to identify and prevent depression among youth, which would substantially decrease the personal, social and economic costs associated with the disorder in these countries and around the globe. The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.