This dissertation used grounded theory methodology to explore the experiences of gender-diverse students in postgraduate education. Thus, the question was asked, "What are the co-curricular and curricular experiences of TGNC students in graduate and professional education programs, and how do they experience a gender-affirming climate for TGNC diversity?" Using a constructivist grounded theory methodology, the researcher interviewed 12 Transgender or Gender Nonconforming graduate or professional students using open-ended questions. In analyzing the interview data, a constant comparative method was used to code and find meaning within the narratives. Following line-by-line, focused, and theoretical coding, a theoretical model emerged, titled the Model for an Affirming Climate for Transgender and Gender Nonconforming (TGNC) Graduate and Professional Students. As a component of the theoretical model, the role of faculty, staff, and the institution were highlighted through a set of theoretical codes: Gender-Affirming Language, Gender-Affirming Pedagogy, Gender-Affirming Policies & Practices, Gender-Affirming Spaces, Gender-Affirming Support Services, Gender-Diverse Representation, TGNC Training & Awareness, TGNC Allyship and Advocacy, TGNC Expansive Curricula, and TGNC Graduate Student Affinity Groups. Higher education administrators can utilize the theoretical model proposed in this study to promote gender-affirming language and gender-affirming spaces, ultimately creating a TGNC graduate and professional student affirming climate. The research posits the possibility of higher education administrators purposefully supporting gender-diverse students by utilizing the Model for an Affirming Climate for TGNC Graduate and Professional Students. Future areas of research are also mentioned, including applying the theoretical model to TGNC students from different racial identities, and exploring the extent to which this theory informs the understanding of the experiences of TGNC students in non-graduate programs, including TGNC students enrolled in certificate programs, associate degree programs, and bachelor's degree programs. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]