The aviator or astronaut is subject to alterations of the environment which may seriously affect his performance or survivability. In a series of experiments, the stresses of hypoxia, vibration, high and low gravity, heat and noise were investigated to determine their effects upon visual function. With the possible exceptions of the latter two, all were found to produce measurable decrements in performance even at moderate levels; however, it is most remarkable that significant capability remains even in rather severe stress. Certain hypotheses regarding these visual decrements that have appeared in the literature are rejected on the basis of experimental evidence.