In the UK, 3 million children are living with a parent with depression or anxiety. These children are 3 times more likely to develop a mental health problem themselves, putting them at risk of later unemployment, physical health problems, and early mortality. This intergenerational cycle of mental illness is partly because children inherit genetic susceptibility to mental illness from their parents, but largely because of the environmental effects of living with a mentally unwell parent. To prevent future generations of children from developing mental illness and experiencing life-limiting consequences, we urgently need to understand how to break this intergenerational cycle. My research will address this by identifying environmental factors that affect whether mental health problems are passed on from parents to children. For example, I will test whether socioeconomic advantage (such as high family income and high levels of parental education) can interrupt the intergenerational cycle of mental health problems. I will also test whether the intergenerational cycle of mental illness is perpetuated by adverse family environments (e.g., child maltreatment, parental conflict, or parent substance abuse), and mitigated by positive social relationships (e.g., family social support, parent-child bond). To answer these questions, I will use large datasets of parents and children from the UK, Norway, Denmark, and the USA. My specific focus will be on parental depression and anxiety (the most common parental disorders) and their effects on child emotional and behavioural problems, which are well established. I will apply cutting-edge research designs and statistical techniques that can show whether environmental factors causally affect the intergenerational cycle of mental illness. For example, to understand the impact of income, I will test whether a cash-transfer intervention reduces the likelihood that parental mental illness is passed onto children. And to understand the impact of parental education, I will test whether the association between parent and child mental illness is mitigated when parents complete extra years of education due to a national education reform policy. These are just two examples of many methods that I will use to examine the causal effects of environmental factors on the intergenerational cycle of mental illness. By examining whether the findings are similar across different research methods, I will draw robust conclusions about which environmental factors can break the intergenerational cycle of mental illness. Crucially, I will use my findings to highlight modifiable targets for interventions to break the intergenerational cycle of mental illness. For example, if high income is found to mitigate the effects of parent mental illness on children's mental health, then economic interventions such as cash-transfers, tax-breaks, or an increased living wage could be employed to prevent at-risk children from developing mental illness. In addition, if unsafe family environments help explain the impact of transmission, then interventions to improve the family environment such as parenting support programmes could be adopted to help break the intergenerational cycle. To ensure that this research has societal impact, I will involve non-academic partners throughout the project, including policymakers from the Department of Health, clinicians, practitioners, and charities. I will also involve young people with lived experience of personal/parental mental illness throughout the research, to ensure benefit for patients. To share findings and co-design interventions to interrupt intergenerational transmission, I will hold a policy workshop with key stakeholders. I will also work with the Centre for Mental Health to conduct an economic evaluation on the cost effectiveness of interventions to interrupt transmission, and present findings to policymakers to convince them to invest in preventative interventions.