The Irish Longitudinal Study on Ageing (TILDA) is a large-scale, nationally representative study of people aged 50 and over in Ireland. It is the most ambitious study of ageing ever carried out in Ireland and represents a step-change in terms of data, knowledge and understanding of ageing with which to inform policy and novel research. TILDA is designed to maximise comparability with other well-established international longitudinal studies. More than 8,000 people aged 50 and over accepted the invitation to participate in the first wave of TILDA, and the majority of these also agreed to undertake a comprehensive health assessment.