The ability to collaborate is an important skill for addressing complex sustainability challenges of the 21st century. Thus, sustainability education needs to build interpersonal competency. This chapter introduces a series of insights, based on empirical evidence and supported by theory, into developing students’ interpersonal competency in project-based sustainability settings. Student interactions in such settings – with peers, instructors, stakeholders, and mentors – typically create opportunities to foster interpersonal competency development for more effective and rewarding collaboration. Interpersonal experiences of collaboration create learning edges for students. In the classroom, facilitated processes of reflection and conversation can help students consciously learn from difficult interpersonal experiences and integrate these learnings into their practice. Instructors can serve as role models, using their own insights to not only support course design but also to inspire students in their ongoing professional development.