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 paper puts forward three basic definitions and the different circumstances in which these can be used. It also reviews key issues to be considered when preparing analyses so that the most relevant definition can be used. It also provides examples of the current (2004) sector size using each definition for comparison, together with a detailed breakdown of the variables used. It also highlights the potential for double counting in some cases and shows which definitions can be used for reporting the size of individual Associations and which to use for reporting the overall sector size.