In this chapter, the concept of quality of life, its evolution, and attempts to achieve consensus in how to understand it are presented. In particular, it examines the concept of quality of life in pediatrics, how it is measured, and the various challenges posed by its assessment in children who encounter difficulties linked to their state of health or young age. Indeed, it is difficult for children under a certain age to provide information about their subjective well-being, even though the definition of quality of life implies collecting the individual’s point of view. The chapter go on to present various tools for measuring the quality of life in preschool-aged children, which involves the participation of parents. Finally, an example of a tool adapted to young preschoolers with autism is presented.