Stress-related psychopathology (e.g. depression and anxiety) affects one in eight children in the UK. It is therefore crucial to identify modifiable protective factors that can promote mental health in young people. Physical activity is a promising target for the prevention of depression and anxiety. However, the extent to which it can reduce the burden of mental disorders in young people is not known. This is due to the breadth of environmental, social, and genetic factors that could explain the relationship between physical activity and psychopathology. Furthermore, it is unclear whether physical activity can promote resilience to psychopathology in children who are at higher risk by virtue of genetic liability or adversity. This project will apply cutting-edge causal inference methods, including G-methods, fixed-effects regression, mendelian randomisation, and twin designs, to test whether physical activity can promote resilience against the development of stress-related psychopathology, using data from large population-based cohorts of children from the UK, Netherlands, Norway, and Finland. By comparing results across different studies and analytical approaches, the project will strengthen causal inferences from observational data and minimise potential biases associated with each method. In turn, the results will allow us to draw more confident conclusions regarding whether interventions targeting physical activity could help to reduce the burden of mental health problems in young people.