This lecture explores the historical and historiographical perspectives of census data and cadastral plans, focusing on their significance for studying economic and social history. It delves into the evolution of census-taking methods, the types of data collected, and their relevance for understanding past populations. The presentation also discusses the role of cadastral plans in urban development, land use patterns, property ownership, and their integration with census information. Additionally, it examines key historical figures such as Le Play, Durkheim, and Riehl, who contributed to the study of family structures using census data.