Longitudinal studies - which follow individuals to collect data about health, lifestyle and environment over a period of months, years or even decades - have helped researchers understand disease risk, inform prevention strategies and improve public health planning. But the majority of large-scale longitudinal research has historically taken place in Europe or North America, meaning many populations remain under-represented in the data. A new generation of longitudinal studies is aiming to better reflect population diversity - not just by recruiting from underserved groups, but by embedding community involvement, local scientific leadership and context-specific questions into their design.