Over the past two decades the concept of quality of life (QOL) has increasingly become a focus for research and application in the fields of education/special education, health care (physical and behavioural), social services (disabilities and ageing), and families. This article summarizes our current understanding of the construct of individual QOL as it pertains to persons with intellectual disabilities (ID). The article's three major sections discuss what we know, what we are beginning to understand, and what we still do not know about the QOL construct. We currently know the importance of the QOL construct as a service delivery principle, along with its current use and multidimensional nature. We are beginning to understand the importance of methodological pluralism in the assessment of QOL, the multiple uses of quality indicators, the predictors of assessed QOL, the effects of different data collection strategies, and the etic (universal) and emic (culture-bound) properties of the construct. We have yet to understand fully the use of QOL-related outcomes in programme change, how to best evaluate the outcomes of QOL-related services, and how to use the concept of QOL to impact public and disability reform. The article concludes with a brief discussion of future challenges related to demonstrating the concept's social validity and positive impact on the lives of persons with ID.