The mortality decline in the Nordic countries started at the end of the 18th century with a decrease in infant and child mortality. It was not until the middle of the 19th century that adult mortality started to fall. Recent research shows that improvements in nutrient supply, medical care, sanitation and nursing, did not take place until the beginning of the 19th century, i.e. considerably later than the start of the decline in infant and child mortality. One possible explanation to the initial decline is that a change in virulence of pathogens did occur at the end of the 18th century, i.e. that the decline was caused by factors beyond human control. There is a general agreement that the decline that followed during the 19th century had multli-factoral causes. The importance of different factors is, however, still debated. To proceed with this debate there is an obvious need to construct more data series of age-specific mortality at the macro level and to make full use of family reconstitution data. The use of advanced statistical methods, such as life event analysis, will also be of crucial importance. Equally important is the quality of the data. Since the Nordic data are said to be very good, which they generally are, too little attention has been paid to the examination of its quality. One such problem is the under-reporting of infant deaths in the church books. It is vital to research on infant and child mortality, as well as on fertility, that those periods and parishes with good data are identified.