The intentions for, and the thinking behind, the Green Building Challenge (GBC) assessment framework and its development are made explicit. This paper disentangles three separate roles for environmental assessment of buildings (stimulating owners to improve a building's performance, informing decision makers during the design stages and delivering objective measurements of a building's impact on natural systems). The author provides a number of significant lessons from the development exercise and discusses further implications and directions for developing environmental assessment methods for buildings. A distinction is made between 'green' and 'sustainable' agendas and their implications for the future development of building environmental assessment methods. This is essential in order to clarify the many roles and applications demanded of these tools. The considerable practical overlap between the 'green' and 'sustainable' agendas suggests that they can be reconciled within a single tool. 'Green' performance is most usefully described in relative terms in comparison to similar buildings in the region, while absolute energy and mass flows are a prerequisite for assessing progress towards sustainability. A fewer number of carefully elected performance measures are required to provide a measure of a building's role in ecological sustainability than to describe its green performance. The GBC process has clarified the roles and applications demanded of different tools and these lessons will be integrated into the restructuring and application of GBTool, and add significantly to the wider debate on environmental assessment. Cet article explique les objectifs et les raisons qui ont amene a elaborer le cadre d'evaluation Green Building Challenge (GBC) ainsi que son evolution. Il precise le role de trois facteurs distincts qui participent a l'evaluation environnementale des batiments: inciter les proprietaires a ameliorer les performances des batiments, informer les decideurs pendant la conception et fournir des mesures objectives de l'incidence d'un batiment sur les systemes naturels. L'auteur tire un certain nombre d'enseignements significatifs de cet exercice et examine d'autres implications ainsi que des orientations relatives a l'elaboration d'autres methodes d'evaluation environnementale des batiments. Il fait la distinction entre les aspects 'verts' et les aspects 'durables' et examine leurs implications dans la perspective de l'elaboration de nouvelles methodes. Cela est essentiel si l'on veut expliciter les nombreux roles et applications demandes a ces outils. Cette distinction est capitale; le chavauchement considerable de ces aspects suggere que l'on pourrait les fusionner dans un seul outil. Les performances 'vertes' sont tres bien decrites en termes relatifs, par comparaison a des batiments similaires de la region, alors que l'energie absolue et les flux de masses sont une condition prealable a l'evaluation du progres vers la durabilite. Un nombre moins important de mesures de performances selectionnees avec soin servira a donner la mesure du role des batiments dans un contexte de durabilite ecologique plutot que de decrire leurs performances 'vertes'. Le processus GBC a precise les roles et les applications des differents outils et ces enseignements serent integres dans la restructuration du GBTool, de ses applications a une plus grande echelle et au debat elargi sur l'evaluation environnementale.