The overall aim of this thesis is to explore the meaning of age in the social world of preschool and age as a discursive resource in daily practices. In relation to this aim, the following specific research questions are raised: (i) How do preschool children practice age categories and what interactional goals are achieved through categorical work in interaction with other children and with teachers? (ii) In what ways is the institutional order of preschool handled and produced through the participants´ categorical work? (iii) How and in what ways are age categories discursive resources in the participants´ socialization practices? Data were generated through a year of field work during which participant observation was carried out among a group of preschool children aged 3-5 years. In line with the ethnomethodological approach of this thesis, age is treated as an interactional accomplishmenttalked into being by children and adults who partake in a variety of activities within the institutional setting of preschool. The data have been analyzed using Membership Categorization Analysis to flesh out the participants' use of cultural and institutional notions of age for a range of interactional purposes. The findings are presented in three chapters where the first chapter makes visible how preschool children use categories for age to achieve different goals in interaction within the peer group and with teachers. Age categorization is practiced in various ways, depending on the goals to be achieved and the age relations at hand. In the second chapter the focus is on the connection between the institutional context and the social interaction within that setting. In the third chapter, the analysis focuses on teachers´ situated socializing practices and age as a rhetoric and discursive resource. To sum up, this thesis shows how common-sense knowledge and notions of age, as well as local age-related discourses and norms, are "locked into place" (Baker 2000:112) in the everyday life of preschool in and through categorical work. By analyzing age, a category that children themselves constantly orient to and make relevant, this thesis broadens existing research in early childhood education on children´s perspectives. This thesis also contributes to methodological expansion in the research field of early childhood education by using Membership Categorization Analysis (MCA), a methodology seldom applied in early childhood education.