This paper introduces the Office for National Statistics Longitudinal Study (ONS-LS). It highlights linkage of the 2021 Census data, and research opportunities it offers to examine a range of new topics, and socio-economic and demographic changes taking place since the 2011 Census. The ONS-LS follows a sample of the England and Wales population from Census data longitudinally (1971–2021), linking administrative data (births, deaths, cancer registrations), with capacity for linkage of data at low level geographies. Members are selected based on four birthdays, providing a 1% representative sample of the population. The study is maintained as a continuous multi-cohort through addition of new births and immigrants with the same four birthdays (members leave through death or emigration). The main strength of the ONS-LS is its large sample size (∼1.3 million), allowing analysis of small areas and specific population groups. It has been used for a variety of research including on: Mode of transport to work. Socioeconomic inequality in place of death. Cancer diagnoses/mortality among ethnic minorities. Outcomes of cared for children. Linkage of 2021 Census data provides opportunities to examine: new topics in the 2021 Census (sexuality, veteran status). longitudinal transitions related to questions first included in 2011 (e.g., language, national identity, passports). changes occurring during the 2011-21 intercensal period when the EU referendum and “Brexit” occurred. Covid-19 pandemic.