The ultimate limitations in flight performance and in future civil air carrier equipment are the limitations imposed by what may be termed “human tolerances.” This is particularly applicable to the matter of the supersonic transport. The discussion of man’s maximum adaptive capacities for the majority of stresses potentially encountered in atmospheric and transatmospheric flights points to the weakest links in the man-machine complex of air and space transportation. An attempt is made to point out the means by which the human tolerances can be maximally adapted to the advanced technology.