The Cambridge City over-75s Cohort Study (CC75C), a longitudinal population-based study of cognition and function in advanced old age, began in 1985 from a survey of over 2,000 men and women aged 75 years and older in Cambridge, United Kingdom (UK). It is one of the largest and longest-running observational studies of aging, as well as one of the first and largest prevalence studies of dementia and its sub-types to be conducted in the UK. The CC75C study originated in the mid-1980s as a survey of dementia prevalence called the Hughes Hall Project for Later Life. This initial study comprised 2,610 men and women aged 75 years and older at enrolment (1985-1987) who were registered with a selection of geographically and socially representative general practices in Cambridge, UK. The sample.s age specific gender distribution and residential status closely matched national population data for that period. From this initial survey of 2,610 people, a baseline sample of 2,166 individuals was selected for the longitudinal cohort. To date, a total of seven interview waves have been completed with survivors and proxy informants. The population has been followed-up to a very advanced age, and the study plans to follow-up individuals until the cohort has no living members. Sociometrics has archived a subset of variables collected from the seven interview waves (data set version 3.0). These variables include demographic variables (age, sex, marital status, education level, type of accommodation), social contacts, service contacts, physical conditions that interfere with day-to-day tasks, activities of daily living (ADL) and instrumental activities of daily living (IADL). This User.s Guide contains detailed information about 127 distinct variables administered to the 2,610 respondents in the full original sample 2,610 respondents and to the 2,166 respondents followed through interview waves 1 through 7.