In the Netherlands, data about the spatial distribution of Dutch citizens structured on the basis of their “migration background” is available in governmental open data sets. In this dissertation, such data are called “race-ethnic” for the combined racial and ethnonationalist connotations of the categorizations and labels. I will argue that the appropriation of race-ethnic data for new purposes enables a process of racialization, understood as the selection of “particular human features for purposes of racial signification” (Omi and Winant 2015, 110). To understand how processes of racialization are not only historically and socially determined, but also result from specific technologies, I propose the concept of technologically mediated racialization (TMR). I use TMR in the investigation of the development of past technologies used for counting and clustering people in the Dutch census between 1899 and 2011. I show how race-ethnic categorization is intimately tied to sociocultural ideas about which human characteristics matter, as well as to the available technologies to record, store, and distribute that information. I will trace the historical connection between contemporary Dutch categories and clusters, such as “person with a migration background”, and older categories and clusters, such as “niet-Westerse allochtoon”, and “vreemdeling”. This will allow me to show that contemporary categorization practices are still informed by Dutch colonial history and migration history. As a result, label changes are both unable to solve racializing practices, and, simultaneously, make historical connections between race-ethnic categories and colonial history harder to access. This process can be understood as a form of colonial aphasia: “a loss of access that may verge on active dissociation” (Stoler 2016, 12). In my first two case studies, I show how race-ethnic categories are made available by the data infrastructure of Statistics Netherlands (CBS) and are used in data applications such as the Allochtoon-o-meter of the ...