Existing estimates of the prevalence of intellectual disability vary substantially between different studies and among different populations (AIHW: Wen, 1997; Roeleveld, Zielhuis, & Gabreels, 1997; Leonard & Wen, 2002), often affected by differences in operational definitions and methods of estimation. For example, in Australia, operational definitions and estimates of prevalence of intellectual disability have been affected by the periodic revisions of the American Association on Mental Retardation (AAMR) definitions and classifications, and by the variations in definitions and classifications between AAMR and other major classification systems. Issues relating to definitions and methods of estimation of intellectual disability are reviewed in AIHW: Wen (1997) and AIHW (2003a). Definitions of these terms are provided in Table 1. The survey covers both rural and urban areas in all states and territories. Data are gathered from both household and institution samples; at the time of the survey the total population of Australia was 18,659,700 people.