This dissertation consists of three comparative studies of health and mortality which address major topics in the field: persistent mortality disparities within the U.S., how mortality in the U.S. compares to other high-income countries, and early life determinants of adult morbidity in developing countries. The design of these studies is predicated on the belief that we can draw meaningful inferences from comparisons across populations. Chapter I examines the contribution of smoking to black-white mortality differences above age 50 from 1980-2005. This study shows that smoking-attributable mortality accounted for 20-40% of the black-white mortality gap among males between 1980-2005, but accounted for almost none of the black-white mortality gap among females. The results support the hypothesis that later initiation and lower rates of smoking cessation among black men may contribute to their higher levels of smoking-related mortality relative to white men. Chapter II provides a comprehensive assessment of U.S. mortality relative to other high-income countries. This study demonstrates that mortality differences below age 50 account for the majority of the gap in life expectancy at birth between American males and their counterparts in other high-income countries. Among females, this figure is 41%. The major causes of death responsible for Americans' excess years of life lost below age 50 are unintentional injuries, noncommunicable diseases, perinatal conditions, and homicide. This study also finds that the U.S.'s unique pattern of age-specific mortality rankings holds for birth cohorts whose mortality experience spans the period 1935-2005. Chapter III explores the association between two measures of early life conditions and adult morbidity in six countries. The findings from this study indicate that those born during the autumn in Ghana, Mexico, Russia, and South Africa and during the monsoon in India experience a health advantage. In China, the autumn-born experience a health disadvantage. This study also finds that pre- and postnatal rainfall and temperature conditions are associated with adult health outcomes, particularly height and blood pressure. The results provide support for the hypotheses that early life disease and nutritional conditions are important influences on later life health.