Almost 50% of all work disability claims in The Netherlands regard young workers (<35 years) with mental health problems. Today, starters in the labour market and young working adults have to earn a living in a new world of work (e.g. more temporary work, multiple jobs) with transformed labour markets (e.g. more self-employment and a 24/7 work cycle in a global economy). This evidently challenges mental health. The available evidence on work and mental health is fragmented, i.e. focused on specific exposures and/or worker groups, and had not turned the lens to life prior to work. We need transdisciplinary thinking that integrates principles from life course theory with public health methods to examine working life trajectories of young adults. In this project, we will conduct in-depth trajectory and life course analyses with 15-year follow-up data from the unique Dutch TRAILS cohort to derive the main working life trajectories of young adults. A new measurement at age 28/29 years will be added to TRAILS to extend these trajectories and to collect new data on mental health and the interaction of work and family life in early adult life. Young adults work functioning will be examined with longitudinal data in the exceptional Dutch three-generational LifeLines cohort. We will use the Danish longitudinal Vestliv birth cohort, enriched with register data, for cross-national comparisons of young adults working life trajectories. To learn how the socio-political context shapes the working life trajectories of young adults in the Netherlands and in Denmark, multi-perspective qualitative data will be gathered from key stakeholders in policy and practice, and from young adults. The project builds upon collaboration with a multidisciplinary international group of experts and stakeholders in policy and practice to identify opportunities for effective policies and practices that sustain a healthy workforce for tomorrow.