Drawing on three cases from a larger (N=169) longitudinal study of student writing development, this article shows how STEM students “rewrote” disciplines to suit their writerly purposes as they moved through their undergraduate years. Students made it clear that the institutional dimensions of disciplines, visible in administrative units or departments that control resources and records, remained visible in their mental landscapes, but they had a much more flexible view of the epistemological dimensions of disciplines. Rather than entering a field as novices aiming to emulate the writing of its experts, they drew on the intellectual resources of multiple disciplines in order to carry out their own projects. The goals and choices of these students suggest that the term new disciplinarity has implications for the ways WID is conceptualized. As theorized by Markovitch and Shinn (2011, 2012), new disciplinarity posits elasticity as a central feature of disciplines, calls the spaces between disciplines borderlands, and affirms the dynamic nature of projects and borderlands with the term temporality. As such, new disciplinarity offers terms and a theoretical framework that conceptualize the intellectual negotiations of students.