Over the past 20 years, there has been a growing interest in the inclusion of health-related quality of life (HRQL) measures to assess the effects of a condition and/or its therapies on a person's health. In response to this interest, methods to assess health status and HRQL have proliferated. There are now a number of valid and reliable instruments available for use in research investigations, which are the culmination of years of research with various populations, and reflect the target populations' perceptions of their health status and HRQL. In this article, we provide a definition of HRQL and its dimensions; describe types of HRQL instruments; discuss variations in how HRQL measures are scored and how these methods influence the interpretation of results; briefly describe utility measures; and summarize by discussing basic issues in the selection of HRQL instruments for research investigations.