Investigating MUltimorbidity ThroUgh cApacity building (MUTUAL) More than a third of adults around the world live with more than one chronic condition. This accumulation of diseases is otherwise known as multimorbidity. Existing studies of multimorbidity have identified that the number of people living with multimorbidity in communities is similar between High (HIC) and Low- and Middle-Income Countries (LMICs). However, very fewer studies have been carried out in LMICs. Not everyone who has multimorbidity share the same diseases, and several different possible patterns are thought to exist. These patterns are normally referred to as clusters, as they represent the clusters formed by different diseases. Most of the research on clusters has been carried out in HIC, with very few studies being conducted in LMICs. Some existing studies carried out in HIC have showed that when a person has both physical and mental disorders, their quality of life is lower, their risk of dying is increased, and their economic burden is higher, compared to a person who only has physical illnesses. This area of work has however been largely under-researched in LMICs, partly for the following reasons: a) analysing multimorbidity clusters is challenging and requires advanced training in data analysis; b) lack of suitable and up-to date data; c) neglect of LMICs mental health research more generally, and d) lack of interdisciplinary researchers trained with appropriate skills to address this complexity, especially in LMICs. With MUTUAL we will develop a platform for high-quality, LMIC-led research to investigate knowledge gaps in our understanding of multimorbidity associated with both mental and physical illnesses (MPM) in LMICs. This will be done by creating a capacity building programme taking place across three countries, in three continents, and with different levels of economic development: Brazil, Ethiopia and Malaysia. Training will be delivered by tutors and mentors in 3 areas of need: a) advanced statistical methods and epidemiology; b) health economics; and c) evidence synthesis. This will be further enhanced by training in transferable skills (e.g. research dissemination, public speaking, leadership, impact, data security). Training resources will be made available outside of the network so that other researchers interested in the training will be able to access it. Early career researchers across the three countries, with support of both UK and in-country tutors, will then work mutually to advance our understanding of multimorbidity using existing datasets of people living in the three countries, with detailed information of health status. This will be carried out using advanced statistical analysis. Health economics techniques will also be used to understand the economic impact associated with specific clusters, to assess which are the most burdensome. Our final objective is to create a network and platform for future training and for further research activities around the topic of multimorbidity. We hope that this project will have a direct impact on people's lives, both individuals living with multimorbidity, but also on early career researchers. 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.