Environmental challenges are often global in scope and require solutions that integrate knowledge across disciplines, cultures, and organizations. Solutions to these challenges will come from diverse teams and not from individuals or single academic disciplines; therefore, graduate students must be trained to work in these diverse teams. In this article, we review the literature on training graduate students to cross these borders. We then present a National Science Foundation Integrative Graduate Education and Research Traineeship Program at the University of Washington as a model of border-crossing graduate training focused on interdisciplinary, international, and interorganizational (I³) collaborations on environmental challenges. Finally, we offer recommendations from this program to those considering similar I³ training programs, including strategies for maintaining faculty buy-in, for scaffolding student training to cross borders, and for conducting focused group trips that give the students structured experience crossing all three borders simultaneously.