The reduction in mortality in this century is as important a watershed for the world as was the industrial revolution. Econometric studies of household sample surveys is contemporary low income countries provide a new basis for estimating causal relationships linking nutrition, mortality, morbidity, and productivity. Improvements in health are also being related to changes in wages, time allocation, fertility and personal incomes, while dealing with both the simultaneous determination of health and income, and certain sources of measurement error. Hypotheses confirmed by microeconometric studies of the determinant of child mortality and fertility are then reexamined with intercountry regressions that seek an explanation for cross sectional and time series variation in child mortality and total fertility rates from 1972 to 1988. Although country aggregate data have clear limitations, they confirm many recent findings of household analyses. Women's education is the powerful engine of demographic change promoting the decline in child mortality and fertility, while slowing population growth. Factors that permit higher levels of calorie consumption per capita, given a country's education and income, are associated with lower levels of child mortality. Combining micro and macro data sources and methods of analysis improves our impressionistic knowledge of the determinants of this century's mortality decline. Progress on this front will provide a firmer basis for modeling fertility behavior, and thus a deeper understanding of the episode of rapid population growth that has marked this century.