Despite growing efforts for Education for Sustainability (EfS), little empirical research documents EfS in the postsecondary classroom and the ways in which sustainability is defined in these classrooms. The purpose of this study was to provide a theory of EfS in the postsecondary classroom in the United States. I sought to understand how postsecondary educators understand sustainability, how they engage in EfS in the classroom, and what contexts influence their work in EfS. Using a constructivist, grounded theory methodology, I selected three institutions - East State University, Liberal Arts College, and Religious University - for this study given both their diversity of mission and their commitment to EfS. I observed 27 different courses, comprising a total of 67 class meetings, and interviewed a total of 42 individuals, 29 of whom were educators, 11 of whom were students, and 2 who were primarily administrators.

Findings revealed that a macro-narrative of sustainability in the classroom concerns (a) the relationship between humanity and the environment, including relationships between communities that are mediated by the environment; (b) the ways in which people come to understand those relationships; and (c) the responsibilities individuals have because of those relationships. Educators described that the term sustainability is complex but also invites dialogue. Variations of EfS in the classroom existed along two continua, one concerning the role of sustainability (whether fundamental or supplemental) and another concerning the teaching framework (practical or theoretical). When used together, these continua created a typology of courses that helped students conceptualize, operationalize, contextualize, or synthesize sustainability. Although variations of EfS existed, a number of pedagogical characteristics were similar in EfS classrooms, including educators' desire to teach beyond content, multiple sources of knowledge, the use of pedagogical partnerships, invitation to conversation among disciplines, and values orientations. These characteristics together demonstrate a path from knowledge to practice in an EfS classroom.

Findings provide a theory for understanding sustainability within the context of a postsecondary classroom and possible variations for EfS in the classrooms. They reveal implications for educators - both inside the classroom and within the co-curriculum - as they provide an empirically grounded theory for EfS.