Loneliness is associated with a range of mental health problems and is a frequently reported problem for young people. Despite this, most loneliness research has been conducted with older adults. Loneliness can be experienced either transiently (occasional or short periods) or chronically (consistent problems developing satisfying social connections). Transient and chronic loneliness may have different effects on health. Little is known about the course and impact of loneliness in young people. This project will assess how different trajectories of loneliness in adolescence are associated with mental health in young adulthood and will assess whether loneliness mediates the relationship between bullying, sexual orientation, and anti-social behaviour and depression in adolescence. From this, we will be able to understand which loneliness types may warrant intervention, what is the most appropriate stage for intervention, and which groups may benefit the most from intervention.