In 1997, the Danish Longitudinal Study of Ageing (DLSA) was jointly initiated by two research institutions responding to the need for a prospective cohort study that would enable longitudinal research on topics related to ageing and demographic change. The DLSA covers a wide range of topics concerning the living conditions of older adults in Denmark, enabling researchers to study social, psychological and health-related processes in ageing from mid-life to older age. With its prospective cohort design and its focus on the ageing process, the DLSA was the first of its kind in Denmark.