This thesis presents an ethnographic study of the role of users in user-centered design. It is written from the perspective of science and technology studies, in particular developments in actor-network theory, and draws on the notion of the assemblage from the work of Deleuze and Guattari. The data for this thesis derives from a six-month field study of the routine discourse and practices of user-centered designers working for a multinational microprocessor manufacturer. The central argument of this thesis is that users are assembled along with the new technologies whose design they resource, as well as with new configurations of socio-cultural life that they bring into view. Informing this argument are two interrelated insights. First, user-centered and participatory design processes involve interminglings of human and non-human actors. Second, users are occasioned in such processes as sociotechnical assemblages. Accordingly, this thesis: (1) reviews how the user is variously applied as a practico-theoretical concern within human-computer interaction (HCI) and as an object of analysis within the sociology and history of technology; (2) outlines a methodology for studying users variously enacted within design practice; (3) examines how a non-user is constructed and re-constructed during the development of a diabetes related technology; (4) examines how designers accomplish user-involvement by way of a gendered persona; (5) examines how the making of a technology for people suffering from obesity included multiple users that served to format the designers’ immediate practical concerns, as well as the management of future expectations; (6) examines how users serve as a means for conducting ethnography-in-design. The thesis concludes with a theoretically informed reflection on user assemblages as devices that: do representation; resource designers’ socio-material management of futures; perform modalities of scale associated with technological and product development; and mediate different forms of accountability.