This essay approaches the history of data by looking at research practices of early modern genealogists. Genealogy was (and is) a data-intensive enterprise. It requires the accumulation, management, organization, and display of vast amounts of personal data reaching back many generations. While recent research has studied in detail the social and political functions of premodern genealogy, and while the staging of genealogical information for purposes of representation has been investigated carefully, next to nothing is known about the work invested in actually researching family lines and dynastic relations. Yet the research strategies and practices, the constraints, and the possibilities of genealogical study had a crucial impact on the shape and understanding of genealogical information. By looking particularly carefully at archives as sites of genealogical data production, this essay attempts to shed light on this understudied question and contributes to the premodern history of data.