Interdisciplinary sustainability programs are emerging globally, but little is known about the learning in these educational contexts. This qualitative case study examined undergraduates' experience in a Sustainability Semester, using the agency/structure dialectic as a theoretical lens. Before the semester, students' models of change for sustainability focused on individual actions. After two years, students' models identified systemic and structural barriers that constrained and enabled their agency as individuals. The discussion examines the "ripple effect" model of change that emerged in early interviews and considers how educators might use the agency/structure dialectic to help undergraduates move beyond individualistic models of social change.