We have reviewed evidence that hypoxic chemosensitivity is variable and that this variation may be both endowed, partly through genetic mechanisms, and acquired, and may reflect fundamental changes in carotid body function. This variation may influence the nature and effectiveness of adaptation to high altitude and to hypoxic disease states such as chronic obstructive pulmonary disease. High chemosensitivity seems to be the choice for coping with the casual exposure to hypoxia; but fundamental, highly effective adaptations, presumably at the level of peripheral tissue, seem to be the strategy of choice for professionally adapted species.