The article profiles a project addressing a central issue in reform-driven STEM education--the need to align educational experiences with real-world STEM. The interdisciplinary program aimed for authenticity by purposefully designing an authentic PBL project and incorporating STEM professionals as volunteer facilitators. These volunteers enhanced authentic elements of the project task (e.g., open-ended task, accountability) and context (e.g., autonomy, collaboration). Volunteers also alleviated pedagogical barriers to PBL--their expertise allowed both volunteers and teachers to take up the facilitator roles foundational to PBL. Professionals who volunteer their time can play vital roles in STEM capacity-building initiatives, including programs designed to broaden participation. Further research is needed to understand how to prepare and utilize volunteers most efficiently in a variety of classroom roles, including the role of PBL facilitator.