Socioeconomic characteristics of the population of the City of Zagreb in the twentieth century changed under the influence of social, economic and political factors. The dissertation deals with changes in the economic activity of the population, the structure of the population by occupation, family and household structures and basic demographic indicators. The main goal of this dissertation is to investigate the dynamics and main factors of these changes and to determine the hitherto unknown demographic sizes of the historical population of the City of Zagreb. Data sources, along with aggregate and microdata of the censuses from 1931 to 2011, include the archival material of the so-called "Consumer registration forms". "Consumer registration forms" are documents at the level of households and their members that were used at the time of rationalized consumption before, during and after World War II. A large sample of digitized "Consumer registration forms" enabled the research of hitherto unknown demographic values for the period immediately before and after the Second World War in the City of Zagreb. In the doctoral thesis, a special focus was placed on the immigration to the City of Zagreb. Statistical modeling examined the relationship between demographic and socioeconomic variables on economic activity, the likelihood of migrant status, and marital status. The main findings of the research on family structures come down to a change in the direction of strong homogenization of family structures after the Second World War, the dominance of the nuclear family and the connection of certain types of occupations with the type of family at the individual level. In terms of economic activity, the biggest changes were recorded in women, with the continuous growth of their economic activity, which by the end of the 1990s became comparable to that of men. Given the size, economic and developmental importance of the City of Zagreb, throughout the 20th century there is a strong interrelationship with the development of the population of the rest of Croatia. Immigration, either as an endogenous factor of the labor market or as an exogenous factor, greatly influenced changes in socioeconomic characteristics: it slowed down the aggregate decline in fertility, somewhat worsened the overall educational structure of the population in Zagreb and had a positive impact on the labor market. population. In family structures, immigration has had the greatest impact in the longer retention of more traditional forms of family structures. The analysis by cohorts and the decade of immigration shows how the immigrant population also adapts to the demographic and socioeconomic patterns of the local population.