Under pre-modern or pre-industrial living conditionsi.e., during most of the history ofhumankindlife expectancy at birth generally varied between 20 and 40 years: mortalitylevels were high and uctuated abruptly, as frequent epidemics, famines and wars decimatedpopulations. Today, although substantial inequalities in health and mortality persistbetween regions and countries around the world, it can be said that, in general, great achievementsin survival have dramatically changed the world we live in. Thus, in some longevouspopulations, the expectation of life at birth currently exceeds 80 years, a truly remarkableachievement.How did that progress occur and what induced it initially? The research studies comprisedin this thesis aim to contribute to the understanding of the process by which lifeexpectancy started increasing in the 19th century, by exploring the characteristics, the dynamicsand the possible causes of the transition from stagnating, pre-industrial mortalitylevels to sustained improvements.Most of the work presented in this thesis relates to the Northern European region. Thefocus on the Scandinavian countries is justied by the unique experience of those populationsin the history of the mortality decline, as they were frequent life-expectancy leaders inthe past. Since the Nordic countries are known for their pioneer role in the collection ofdemographic data for the entire population from the 18th century onwards, a rich historicalperspective can be gained by studying the evolution of the average expectation of life inthose countries, such as the possible factors or mechanisms that rst provoked sustainedmortality reductions in a pre-industrial setting.Given unprecedented and rapid urbanization in nineteenth-century Europei.e., a contextmarked by abysmal dierences in health between low-mortality rural and high-mortalityurban populationsthe relationship between mortality levels and urbanization is given specialattention in the work presented here. In the historical context covered by thesis, anyattempt to understand the evolution of life expectancy is incomplete without a careful examinationof the crucial role of urbanization and its impact on population health. Because ofthis, one of the research studies included relates to Scotland, a population with signicantlyhigher urbanization levels than the Nordic countries during the 19th century.Among the main ndings of the three research studies comprising this thesis (PapersI to III), it is shown that: I) while Norway, Sweden and Denmark went through all fourhistorical phases of the Health Transition (i.e., the model used as the conceptual frameworkin this study), Iceland and Finland had a dierent experience of that process due to the lackof some contextual factors; II) the 1890s where an important period in terms of populationhealth and mortality in Denmark, as it marked the passage from modest to accelerated gainsin life expectancy, especially in the urban areas; III) in a context of large health dierencesbetween low-mortality rural and high-mortality urban areas, the rapid urbanization process(i.e., the increasing weight of the urban population) observed in 19th century Scotland hada negative eect on the changes in life expectancy at the national level.To sum up, the results of the studies presented in this thesis indicate that a particularcombination of factors may explain the earlier phases of the modern rise in life expectancyin the Scandinavian countries, i.e., the passage from pre-modern, stagnating life-expectancylevels to sustained moderate gains. Moreover, the urbanization process and the changes inurban mortality were found to be important factors for understanding the evolution of lifeexpectancy in the study countries during most of the 19th century, as well as the passagefrom modest to accelerated improvements in survival in the last decades of that century.