The sustainability movement in higher education has made considerable headway in the areas of research, campus operations, and community outreach, but has been less successful in bringing about curricular reform. To promote greater thinking about sustainability in the undergraduate curriculum, this essay explores three main questions: What are the implications of sustainability for higher education? What are some noteworthy examples of institutions incorporating sustainability into the curriculum? And, what can we learn from their experiences? The author advocates implementation of a "third order" learning model, emphasizing deep learning, a participative process which takes the form of continual exploration through practice.