As the concept of quality of life (QOL) has evolved from a theoretical concept to both a measurable construct and an action-oriented change agent in the field of intellectual disabilities, there has emerged a corresponding need to develop, implement, and use a systematic approach to the assessment of domain-referenced quality of life outcomes. The purpose of this article is to suggest eight principles based on published literature and the authors' experiences that should underlie the assessment of QOL-related personal outcomes in the field of intellectual disabilities. Data from the development of the "personal outcomes scale" are used to exemplify each principle. The article's premise is that model development and test construction should work in tandem, and that a "quality of life assessment instrument" should be based on an empirically derived QOL conceptual and measurement framework/model.