Ventilation in buildings is necessary first for hygienic reasons and also to preserve the building structure. This is more essential, today, because the buildings are more and more airtight, mainly due to energy regulations. It is also evident that air renewal energy losses and fan consumption become more and more important in relation with the total energy consumption of buildings. Nevertheless, many defaults are encountered on installed ventilation systems. It seems necessary to check the installations, at the starting up and regularly in time, and not only when the problems occur. In France, today, there is no obligation for regular inspections of ventilation systems, whereas it is a legal requirement in other countries like Sweden. DIAGVENT method has been established on the basis of many inspections on different residential and commercial buildings, and on the Swedish experience which where applied in France. This method is described in a small practical guidebook (around 30 pages). It includes three levels of inspections or diagnosis: - DIAGVENT 1: no measurements, only visual checking, for commissioning new installations. The main objective is to verify that the installed system is in accordance with the expected one. The system is started on ; - DIGVENT 2: is the main part in the method ; it is a more detailed inspection, both for new and existing installations ; it includes not only visual checks but also performances measurements (total and local air flow rates, pressures, electrical power) ; it also includes analysis of the results and indications for feasible improvements or more detailed investigations, if necessary ; - DIAGVENT 3: corresponds to specific measurements, when a strong problem has been revealed after DIAGVENT 1 or 2, or after a complaint from the users. It may lead to a very detailed inspection and may include sophisticated measurement techniques (for acoustics, air leakages, air pollution transfers, etc.). Specific measurements are not detailed in the guidebook, but it is shown, depending on the nature of the problem, which point should be checked and what kind of measurement or analysis should be made. DIAGVENT guidebook should be a practical tool for the professionals: engineering consultants, inspectors, installers, maintenance companies It gives, for example, practical information on standard measurement devices for DIAGVENT 2: which type of device and where it can be bought. It gives some useful reference values (air flow rates, duct air leakages, fan electrical consumption) to help the professional during the analysis of the results. This guidebook can be free downloaded on the CETIAT website. French professional training bodies are already interested in including this ventilation diagnosis method in their training programs.