Professional development of graduate teaching assistants (GTAs) regarding interpersonal boundaries is key not only to the well-being of the GTAs but also to the undergraduates they are teaching. GTAs who are developing their professional identities are a primary contact for undergraduates, especially in lower-division classes, and thus play a key role in undergraduate student academic success and retention. Providing guidance for GTAs on how to establish boundaries is critical to support their teaching and research responsibilities and to advance their professional development. Referencing qualitative data from surveys and pre- and post-workshop reflections, this chapter summarizes the GTA programs and professional development workshop materials and explains the connection to an understanding of boundaries--between GTAs and students as well as faculty and peers. These workshop materials translate broadly into instructional support activities such as orientations, pedagogy courses, consultations, and mentoring. The overall focus of this chapter, through specific examples and curriculum, will be on ways that faculty and staff mentors can guide graduate students toward forming a scholarly, reflexive identity as future faculty.