One in ten children in the UK are affected by a mental health problem, causing significant distress to them and their families. Where these problems endure, they can hold children back from reaching their potential in school and the workplace, and from experiencing good physical and mental health into adulthood. Financially, the personal cost of mental illness is £41.8 billion per year in England. In light of this burden to children, families, and society, there is a pressing need for a pathway that can prevent mental health problems as early as possible. We now know that many of the factors that shape risk and resilience to mental health problems have their roots in the first years of life. Children who start off more vulnerable can go on to develop initial difficulties, which can then progress into more established problems. Developing better ways to identify which children and families are likely to benefit from support would help professionals to work with families to take a proactive approach early on. By supporting families to provide responsive, consistent care, we can help to build a strong foundation for mental health. Doing this in the first years of life, when children's development is especially responsive to their early experiences, relationships, and environment, could unlock huge potential to shape the course of children's long-term mental health. Research also suggests that investing early makes economic sense as children are less likely to need more intensive supports later on in life. This promise of a strong start in life has made children's first 1001 days a global health priority, as reflected in the recent World Health Organisation 'Nurturing Care' framework for early childhood development. Yet the insights we have from decades of research in child development have not translated into the public health strategies we need to promote early mental health in the UK. There are two critical factors underlying this gap. Firstly, we lack a way to identify early risk and resilience for mental health problems in very young children that is quick, effective and acceptable to families and professionals. Secondly, early childhood programmes that show promise in preventing problems when they are tested in controlled research studies typically fail to show the same success when they are delivered in real-world services. Although these programmes have been carefully developed they are often too complicated and expensive to deliver at scale. This fellowship will use cutting-edge techniques in epidemiology and data science to develop a tool to identify early mental health needs in very young children and a pathway for more personalised supports. It will bring together the best evidence available from previous studies of early interventions so we can identify which practices and strategies in these programmes tend to be most effective. Stripping these programmes back to their most important building blocks will allow us to work together with families and professionals to redesign how they are delivered so they fit better into family life, respond to families' needs and priorities, and are feasible and practical to deliver. This will be done by testing different approaches out quickly, figuring out what does and doesn't work, and adapting the approach based on this learning. We will do this in the UK as well as undertaking initial piloting in South Africa to ensure the principles and approaches we develop are flexible and can be adapted appropriately to different resource and cultural contexts. The ultimate goal of this research is to co-develop a flexible prevention pathway for early mental health problems that is relevant to the challenges facing families and communities and is responsive to the needs of family life and the services in which they are delivered. This research has the potential to provide the breakthrough impacts needed to change the course of children's mental health.