The Office for National Statistics Longitudinal Study (ONS-LS) is a very good data source for research on household and family composition because it contains data not only on its members, but also on the "non-members" – that is, the people who live in the same household as the ONS-LS member at the time of the relevant census. After reading this guide you should be able to ascertain whether the ONS-LS is a suitable data source for your household/family study, explain what counts as a 'household' and a 'family' in the ONS-LS and determine the structure of complex households and families, as far as is possible given the information obtained from the census.