A review of existing studies which defined and measured healthy ageing as an outcome was undertaken to clarify the term for the purposes of informing policy development and further research into positive health outcomes in older age. Studies which measured the prevalence of healthy (or successful) ageing in population-based settings were identified from a search of health and gerontology databases. Eighteen studies met the selection criteria. The arbitrary nature of the definition, populations sampled, domains selected and measures within the domains resulted in considerable variation between the studies in the proportion of the study population classified as `healthy ageing', which ranged from 3% to 80%. The present review shows the need to establish a standard for defining and quantifying the concept of healthy ageing. Despite the differences, there was consensus in the studies that the multidimensional, positive health outcome should measure the capacity to function well and adapt to environmental challenges in domains assessing physical, mental and social well-being.