Throughout the lifespan there are certain developmental transitions that appear to be particularly important for determining a person’s mental health. The transition from childhood to adolescence is especially important, as many serious mental health problems, such as depression and substance abuse, emerge for the first time during or after this transition. In my research group, we use a developmental psychopathology approach to understand how children and adolescents are affected by the environments in which they grow up. We have especially focussed on how family interactions and other aspects of the child’s environment that have been shown to increase risk for mental health problems (e.g., stress, abuse, socio-economic disadvantage) influence the child or adolescent’s emotional functioning and the development of the biological systems that undergird these emotions. The aim of this work is to not only shed light on the underlying causes of mental health and ill-health during these stages of life, but also to inform innovative approaches to early intervention and prevention by utilising this knowledge to generate and test novel, developmentally-targeted clinical and public health interventions.