Project-based sustainability courses require and facilitate diverse interactions among stu-dents, instructors, stakeholders, and mentors. Most project-based courses take an instrumental approach to these interactions, so that they support the overall project deliverables. However, as courses primarily intend to build students’ key competencies in sustainability, including the competence to collaborate in teams and with stakeholders, there are opportunities to utilize these interactions more directly to build students’ interpersonal competence. This study offers insights from project-based sustainability courses at universities in Germany, the U.S., Switzerland, and Spain to empirically explore such opportunities. We investigate how students develop interpersonal competence by learning from (rather than through) their interactions with peers, instructors, stakeholders, and mentors. The findings can be used by course instructors, curriculum designers, and program administrators to more deliberately use the interactions with peers, instructors, stakeholders, and mentors in project-based sustainability courses for developing students’ competence to successfully collaborate in teams and with stakeholders.