The Human Factor establishes a foundation for the study of ancient demography in the Iberian Peninsula, focusing on its largest province, Hispania Citerior/Tarraconensis. The authors take a multidisciplinary approach, compiling archaeological, epigraphic, architectonic, osteological, and genetic datasets. This comprehensive and detailed study of a single province is necessary to generate accurate demographic estimates and to compare it with datasets from other regions and historical periods. By examining the province of Hispania Citerior/Tarraconensis in depth, the authors provide a detailed understanding of demographic patterns, urbanism, and urbanization rates over time, and link them with the social, cultural, and economic factors that affected the Iberian Peninsula and the Western Mediterranean from the fourth century BC until the end of the Roman period. For instance, population size was a significant indicator of economic growth and performance, and the distribution of people between urban and rural areas played a vital role in the negotiation of collective identities. Additionally, human mobility promoted cultural change and mediated information and technological flows.This is the first comprehensive , state-of-the-art demographic analysis of the Iberian Peninsula from the Iron Age down to the end of the Roman period, and the authors'' integration and interpretation of data provide cutting-edge research and methodology, and fill a gap in the scholarly literature, allowing for a more nuanced understanding of the ancient Mediterranean.