Assessment of the impact of diseases and their treatment on a patient with chronic disease is especially important. When there is no possibility to recover, patients and their closest relatives have to accept changes in lifestyle, which may vary with time. Health professionals assume that improvements in symptoms show better quality of life of patients. Quality-of-life assessments incorporate not only the impact of illness and treatment on physical functions, but also its effect on lifestyle and emotional well-being. Quality of life deals with a higher order of complexity: the impact of functional impairment on other aspects of life, e.g. the ability of children to go to school or play, and the emotional effect of these restrictions. The aim of this article is to present measurement tools and how to use them and the application of quality-of-life measurement in pediatric medicine. Every investigator has to raise the question to himself/herself, "What is health-related quality of life," and before planning the investigation, answer this question correctly.