Accidents are taken into account in risk assessment and shall be a basis for knowledge to be considered for improving the safety level at the design and operation stages in various industries and activities (transportation). Unfortunately, in many cases, detailed data are difficult to be accessed. Lessons learnt from French accidents in plants and transportation will be developed. In the detailed investigation of accidents, emphasis should be given on the need to consider product(s), process, causes, circumstances, evolution (chain of events) and consequences. Some examples will be developed on involved materials and products, on accidental phenomenon, on the general design of a plant or transportation system, on the reliability of safety related equipment, on procedures and on operator's influence.