Over the coming decades the world will face a wide range of complex, new and persistent public mental health challenges, exacerbated by disruptive events, many of which can be fully addressed only through strategies and investments that improve lifelong public mental health outcomes for everyone. 1 in 6 adults in England have a common mental health condition. Mental distress and ill health are associated with significant disability, sickness absence, unemployment, and suicide attempts. Three quarters of all mental health conditions have occurred in young people by the age of 24 years. Despite widespread acknowledgement that mental distress and illness make a substantial contribution to the global burden of disease, there is still a major gap in evidence to inform policy making for their primary prevention. We will establish the 'Prevention of Risks and Onset of Mental Health problems through Interdisciplinary Stakeholder Engagement' (PROMISE) Population Mental Health Improvement Cluster, which will create new opportunities for population-based improvements in mental health. We will focus on three challenge areas: 1. Children and young people; 2. Suicide and self-harm prevention; 3. Multiple long-term conditions. Challenge areas will be supported by four cross-cutting themes: 1. Partners in policy, implementation and lived experience; 2. Data, linkages and causal inference; 3. Narrowing inequalities; 4. Training and capacity building. We will work with stakeholders across public health, local government, voluntary organisations and interdisciplinary academic experts, and people with lived experience of adversities which impact mental health, to identify and rigorously evaluate population-level interventions which hold the greatest promise for the improvement of mental health. The structure of our cluster reflects the integration of academic, policy and lived experience in leadership and delivery, which will lead to systems change and the ability to work effectively across traditional silos which have held progress in this area behind. We will use a range of large-scale datasets, including representative studies which follow people over time, nationally representative studies of health, and data generated when people come into contact with health and other services, take part in census, alongside information from children in schools, and the linkages between these, for our investigations. We will use statistical methods in this data to understand which population-interventions benefit people's mental health and reduce inequalities. We will draw on the wide-ranging interdisciplinary expertise of our team to develop a unique suite of training (seminars/ tutorials/ short training videos) which will be freely available, the training will be entitled "New ways of working in population mental health" and will cover a range of topics useful to researchers, practitioners, and people with lived experience. Our cluster will foster methods to develop creative and innovative solutions by working with people who have not applied their expertise to improving population health before, through a range of approaches:1. We will work with arts-based practitioners to develop creative outputs (films, children's books/ comic strips, animations, infographics, public photography and art exhibitions), which will also ensure inclusive engagement. 2. We will convene a series of interdisciplinary 'sandpit' events to engage a wide range of interdisciplinary groups, to develop innovative projects across challenge areas. 3. We will convene policy roundtables with support from English, Scottish, Welsh and Northern Ireland Government representatives, to bring together stakeholders, experts, policymakers, and the public to engage in discussion on cluster challenges to gather feedback, build consensus, and develop actionable recommendations. Our findings will be co-produced with people with lived experience.