Doctoral dissertation by Henriette Tolstrup Holmegaard. This dissertation is a longitudinal qualitative investigation of student choice of higher education in general, and transition into science, technology, engineering and mathematics (STEM) in particular. The dissertation offers a coherent understanding of choice, transition, and retention in terms of perceiving the construction of student identities as continuous processes. The dissertation shows how choosing what to study are social processes in where various interests constantly are negotiated and personalized. Difficulties associated with encountering higher education STEM are accompanied by a sense of a gap between student expectations to the study programme, notions of attractive identities and their actual experiences. The dissertation show how students apply different negotiation strategies to bridge the gap. The implications of this study are discussed in terms of how to support students to meaningfully construct attractive STEM identities.