The Office for National Statistics Longitudinal Study (ONS-LS) is a 1% sample of the population of England and Wales. It contains linked census and vital events data, beginning with a sample from the 1971 Census. This guide provides information relating to: deaths of LS members; widow(er)hoods; infant deaths to ONS-LS sample mothers; and still births to ONS-LS sample mothers. This guide aims to: inform potential users of the LS of the scope of mortality data available; describe these data and the system for coding causes of death, including changes over time (International Classification of Diseases (ICD) revisions 8, 9 and 10); explain how underlying causes of death are selected and detail other variables that may be used, along with any restrictions; and give some simple examples of how LS mortality data can be recoded, described and analysed.