International audienceSince its beginning in the mid 1940s, human factors has tried to keep up with the ever increasing demands from technological and societal developments. Looking back, the development of human factors can be described as corresponding to three ages. In the first age, humans were seen as too too imprecise, variable, and slow to allow the full use of the technological potential. In the second age, humans were seen as failure prone and unreliable, hence a challenge to system safety. In both ages, the human was treated as an entity, as a part that could be described independently of the whole. In the third age, humans are recognised as being necessary if work systems are to be safe and productive. Human performance variability is accepted as the necessary basis for effectively coping with the complexity of the work situations and system performance is understood as the non-trivial result of interdependent parts