The PASSYS project was formed in 1986 by the Commission of the European Communities with the aim of increasing confidence in passive solar heating systems. One of the ways chosen to do this was the approval/development of a European validation methodology for building energy simulation programs. The article summarizes the findings from the work of the Model Validation and Development Subgroup within PASSYS. A detailed description of the work may be found in the following two references: S.O. Jensen (ed.), The PASSYS project phase 1, Subgroup model validation and development, Final report, 1986-1989, EUR 13034 EN, Commission of the European Communities, Directorate General XII for Science, Research and Development, Brussels, Belgium, 1990; and S.O. Jensen (ed.), Validation of building energy simulation programs, Vols. 1 and 2, Research report PASSYS subgroup model validation and development, EUR 15115 EN, Commission of the European Communities, Directorate General XII for Science, Research and Development, Brussels, Belgium, 1994.