The 1990s have seen a remarkable decrease in mortality among infants and children in most developing countries. In some countries, particularly in sub-Saharan Africa, these declines in mortality among children have slowed and are now increasing again. Internationally comparable data derived from survey programmes, such as the Demographic and Health Survey (DHS) programme, are available both to document the changes that have occurred in mortality and to provide insight into some of the factors that may explain these trends in mortality. The factors found in repeated DHS programmes that explain these trends fall into five categories: fertility behaviour; nutritional status, breastfeeding, and infant feeding; the use of health services by mothers and for children; environmental health conditions; and socioeconomic status. Both simple analyses and multivariate analyses of changes in these factors between surveys indicate that all factors affected the mortality trends. However, to explain trends in mortality, the variables themselves had to have changed over time. During the 1990s fertility behaviour, breastfeeding, and infant feeding have changed less than other factors and so would seem to have played a smaller role in mortality trends. This study confirms that trends in mortality during the 1990s were related to more than just a handful of variables. It would, therefore, be a mistake to concentrate policy actions on one or a few of these while forsaking others. Countries with the largest decreases in mortality have had substantial improvements in most of the factors that might be used to explain these changes. In some countries mortality has risen. In part these increases can be explained by the factors included in this study, such as deterioration in seeking medical care for children with fever. Other factors that were not measured, such as the increasing resistance of malaria to drug treatment and the increased prevalence of parental HIV/AIDS, may be contributing to the increase noted.