The debate about the greenhouse effect has strongly highlighted the necessity to base the negotiations on consistent and comparable evaluation of energy efficiency situations in the various countries, Until recently, no consensus among countries could be established for assessment which could serve as a reference for cross-country comparison, It is the reason why national agencies in charge of implementing energy efficiency policies are much concerned with this gap, In collaboration with 12 European agencies, Ademe took the initiative to launch such a project which was supported by the EC (Save program), The way of implementing such a decentralised project is of a fundamental importance if the results are to be endorsed in the future by the member states. The methodology used was agreed among all participants, It refers to ＇the economic approach＇ using the energy intensity ratios and the ＇technico-economic approach＇, About 600 comparable energy efficiency indicators, both descriptive and explanatory, have been defined, calculated and interpreted on a common basis, They are stored in the ODYSSEE data-base, This article will discuss the methodological issues encountered in the implementation of the project, This article could not have been carried out without the extensive collaboration of 16 national teams and the support of the SAVE project (DGXVII).