Health-related quality of life (HRQOL) has become one of the important endpoints in cancer treatments. However, a relatively small proportion of oncologists truly understand the concepts and uses of QOL assessments. In this article, I discuss psychometric properties that should be verified with QOL instruments, scope of the QOL concepts that should be assessed in oncology, how to choose the appropriate measures, and cross-cultural validation and statistical issues. Several examples of QOL assessments in recent clinical trials in oncology are also reviewed.