A number of definitions of the size of the Housing Association (HA) sector have been applied in the past, depending on what it was being used for. This project grew out of the need to review these definitions and to understand the key issues to be considered when preparing analyses so that the most relevant definition can be used. It also reviewed the potential for double counting and shows which definitions can be used for reporting the size of individual Associations and which to use for reporting the overall sector size.