The difficulties associated with the calculation of a factor of safety aiming at assessing stability conditions of slopes in clay are well known. More than thirty years of experience in the Province of Quebec have shown an important variability in the geotechnical investigations performed and in their conclusions concerning stability, depending on the people involved. With the objective to standardize these investigations, the Quebec Government is preparing a manual for evaluating of the stability of slopes in post-glacial marine sensitive clays. When geological conditions are relatively uniform, as it is the case in most clayey deposits in Quebec, the observations which can be gathered in situ and from existing documents allow the determination of a number of indices that can be used for classifying the slopes in terms of stability. The manual first recommends a qualitative evaluation of stability conditions based on some criteria without, in most cases, calculating a factor of safety.