The Flemish government considers well-being of pupils as an important output indicator for the quality of education. The education inspectorate needed an instrument to measure this well-being in a school context, an instrument that should also be a basis for actions plans aimed at enhancing pupils' well-being. The development of this instrument is described in this article. A total of 342 pupils were interviewed. The material from these interviews was used for construction of--mainly Likert-type--items for a questionnaire. A pilot version of the questionnaire was examined by experts and tested with a random sample of 306 pupils. The experimental version was tried out on a random sample of 2054 pupils. This resulted in a questionnaire with four consistent scales and thirteen subscales. Significant differences between schools were found. Atmosphere at school, contacts with teachers, involvement in class and at school, school regulations and infrastructure were among the best predictors.