Graduate training in sustainability science (SS) fo- cuses on interdisciplinary research, stakeholder-researcher partnerships, and creating solutions from knowledge. But be- coming a sustainability scientist also requires specialized training that addresses the complex boundaries implicit in sus- tainability science approaches to solving social-ecological system challenges. Using boundary spanning as a framework, we use a case study of the Sustainability Solutions Initiative (SSI) at the University of Maine to explicate key elements for graduate education training in SS.We used a mixed-methods approach, including a quantitative survey and autoethnographic reflection, to analyze our experiences asSSI doctoral students. Through this research, we identified four essential SS boundaries that build on core sustainability competencies which need to be addressed in SS graduate pro- grams, including: disciplines within academia, students and their advisors, researchers and stakeholders, and place-based and generalizable research. We identified key elements of training necessary to help students understand and navigate these boundaries using core competencies. We then offer six best practice recommendations to provide a basis for a SS education framework. Our reflections are intended for aca- demic leaders in SS who are training new scientists to solve complex sustainability challenges. Our experiences as a co- hort of doctoral students with diverse academic and profes- sional backgrounds provide a unique opportunity to reflect not only on the challenges of SS but also on the specific needs of students and programs striving to provide solutions.