Health-related quality of life (HRQoL) measurements have become increasingly important in brain tumor research, next to traditional outcome measures such as overall and progression-free survival. Several validated questionnaires have been developed to measure HRQoL in clinical trials, as well as in clinical practice. In brain tumor patients, both the tumor and treatment may have an impact on HRQoL, which can be positive and negative. When determining the net clinical benefit of a new treatment strategy, both the quantity and quality of life (QOL) should be considered. Because treatment may benefit or harm both quantity and QOL, a trade-off discussion may arise when these two outcomes are conflicting.