There are key transitions that often occur when adolescents become adults, such as leaving full-time education, starting a full-time job, living with a partner, or becoming a parent. In previous generations, when these transitions happen early or happen close together, they have been shown to be related to poorer health and related behaviours, for example, weight gain or increased smoking. However, there is little evidence available on what typical transition patterns look like for today’s young people, which patterns are the most harmful to health, or the reasons that these patterns cause poorer health (for example, is weight gain in those who have made lots of these transitions early in life explained more by the extra stress or by lack of time to eat healthily or exercise). Using data from two recent UK birth cohorts, and sophisticated methods of analysis, we will try to answer these questions. Where possible, we will see if findings differ for sex, ethnic minority, and LGBTQ+ groups. This information can help us understand the best way to support young people moving from adolescence to young adulthood, to help optimise their health later in life.