The authors have validated three quality of life questionnaires designed to assess mostly subjective aspects of quality of life, in infants (QUALIN questionnaire, a parents' report), children (AUQUEI questionnaire, a pictured self-report for children aged 3-11 years old) and in adolescents (OK.ado questionnaire). Psychometric properties being satisfactory, these questionnaires were completed by different samples, in different health states. We were thus able to detect the influence of several factors on children or adolescents' quality of life: illness tends to alter quality of life, but this is also the case for several psychosociological conditions, especially when combined with the child's age. In adolescents, one has to be careful in interpretation of results, since the healthy adolescent tends to complain to adults.