Expectation of life at birth provides a simple measure of the state of health of a country. Differences in the expectation are examined in the United States over time and between males and females and Whites and non-Whites, and worldwide between market and nonmarket developed countries, and between developed and developing countries. The principal factors responsible for the trends and the current differences are changes in the social and physical environment, in personal behavior, and in medical care, and their relative importance is assessed. It is concluded that, at present, the principal environmental hazards worldwide are those associated with poverty of individuals within the market economies and of communities in the developing countries and that in the future, they will be the effects of overpopulation and the production of greenhouse gases.