College majors are important stratification mechanisms, shaping occupational trajectories, postcollegiate earnings, and patterns of inequality by gender, race, and class. Yet the processes through which students and fields of study come together have received limited theoretical and empirical attention. We advance a theory of major selection as a form of organizational differentiation that both builds on and mediates parallel processes earlier in the life course. Our longitudinal conceptualization links the development of academic subjectivities in K-12 education with postsecondary organizational conditions. Compared with their prior schooling, college students exercise greater agency over their course-taking, face greater ambiguity about the purposes of school, and confront complex curricular choice architectures. These conditions are sustained by an organized anarchy in the provision of courses and majors that simultaneously enables and constrains selection. Our theory recognizes the cumulative and contingent character of major differentiation and illuminates enduring problems of stalled college progress.