The Office for National Statistics Longitudinal Study (ONS-LS) is the largest longitudinal study containing data on ethnicity in the UK and can therefore be used to examine ethnic differences in many health and social outcomes, which are difficult to examine in other longitudinal studies owing to insufficient numbers. This guide aims to ascertain whether the ONS-LS is a suitable dataset for your ethnicity study, specify the sample population for your study and specify the variables that you will need to extract from the ONS-LS for your study.