Between 20 and 30 million people live at altitudes above 2500 m and if all mountain dwellers are considered, the number would increase greatly, but still this number is a small proportion of the total population of the world. However, if mere numbers do not justify detailed study of these populations, the fact that high altitude people—and to a lesser extent all mountain dwellers—inhabit a narrowly specialized environment warrants that their responses to the stress components of this environment be studied comprehensively. The insights gained from such studies, apart from their intrinsic academic interest, may eventually help identify appropriate intervention points for maintenance of fertility at optimal levels, control of mortality and morbidity, and, in general, for promotion of the population health of these generally remote populations. The World Health Organization (WHO) (1971) has defined health in terms of “complete physical, mental and social well-being and not merely the absence of disease and infirmity.” However, we shall be concerned in this chapter with the demographic variables of population growth, fertility, and mortality, that is, the ultimate indicators of well-being.