When it comes to what higher education students know about sustainability, where they learn matters. In this study, we explore the extent to which students' level of sustainability knowledge differed according to where they previously learned about the environment. In an online survey administered to undergraduate students enrolled at Michigan State University, a large university in the Midwestern region of the United States, we found a significant relationship between students' level of sustainability knowledge and their environmental learning source. Environmental knowledge gained in the classroom, both at the secondary and postsecondary levels, had the strongest (positive) influence on students' present sustainability knowledge, while there was a significant (negative) relationship between how frequently students gathered knowledge from their parents and their level of knowledge. Results from this study suggest that instructors need to be intentional about the types of prior knowledge they use as a springboard when teaching students about sustainability.