[The exposure of man to altitude when flying: from Paul Bert to today].

This paper is organised around Paul Bert's inputs to modern aviation physiology. Paul Bert gave a masterly historical work, which established the notion of oxygen partial pressure as the right physiological parameter for the body. Human protection against aviation altitude takes this notion into account, by modifying either the pressure surrounding the body or the oxygen fraction in inhaled gases. We also consider the case of protection against accidental loss of aircraft pressurization.