A large body of research has shown that adverse experiences during early childhood can affect children's development and have a lasting impact on their mental and cognitive health. This evidence suggests that the first few years of life form a window of vulnerability, or sensitive period, for adversity. This research has contributed to effective policy interventions designed to protect young children from adversity and improve their well-being. Inadvertently, the focus on early childhood has also led to a neglect of another formative period of our lives - adolescence. Adolescence is characterized by protracted changes in brain structure and cognition. At the same time, young people experience profound changes in social roles. They become increasingly independent from their parents, and more focussed on peers. Emerging theories of adolescent development suggest that these social, cognitive and neural changes in adolescence may give rise to a second sensitive period after early childhood - particularly for adverse social experiences such as being bullied or excluded. We are now at a unique point in time where recent advances in statistical methodology, combined with emerging data following large cohorts of young people over time, allow us to test these theories systematically. This provides unprecedented opportunities to foster well-being in young people by informing adversity prevention, detection and intervention efforts. The three main objectives of our research are: 1. Identifying different types of social adversity To capture the multifaceted nature of social adversity, we have established a partnership with the Children's Charity Barnardo's. We will be able to use their data on social, educational and mental health practitioners' views of what types of social adversity matter most to young people. Guided by these insights, we will analyse data from large UK cohorts to understand how different types of social adversity affect mental health and cognition in young people. We will feed our insights back to practitioners to help guide their practice and support young people who experience social adversity. 2. Characterizing windows of vulnerability in adolescence Leveraging data from large UK cohorts, we will analyse how social adversity affects children, young people and adults. This will help us understand at what point in life what type of adversity affects us most. It will also tell us whether there are sensitive or vulnerable periods in adolescence. This work will help inform policy, charity and government work, by highlighting at what point in life prevention and intervention efforts may be most important. 3. Investigating developmental sequences For this objective we will analyse data on young people's experience of adversity, as well as information on their cognitive and brain development. This will allow us to understand the sequence of developmental events leading from adversity to cognitive changes. This will generate new theories about sensitive periods in adolescence and inspire future research into adversity. Our work will lead to a new understanding of vulnerabilities in young people. It will pinpoint what types of social adversity need to be targeted, and at which point in life, in order to ameliorate cognitive and mental health problems later on. To ensure that our findings reach young people and those caring for them, we will write an accessible policy brief in partnership with Barnardo's, and host a collaborative workshop for practitioners, policy makers and charities. In this workshop, we will discuss our findings and seek input from stakeholders on how our findings synergize with policy and practice. We will then take our findings to the public through online, print and social media, as well as presenting our work in schools. All of this ensures that our work has a lasting and positive impact on young people's well-being.