In many higher education institutions, graduate teaching assistants have played an important role in undergraduate education (Luo, Bellows, & Grady, 2000). Twenty five percent to 38% of undergraduate instructions are performed by graduate teaching assistants (Nyquist, Abbott, Wulff, & Sprague, 1991). In exercise and sport science area, graduate teaching assistants teach most of basic instruction program courses. However, research on graduate teaching assistants' teaching experiences and their challenges has not been widely explored. The purpose of this study was to explore the experience of graduate teaching assistants who have taught for two years at the doctoral/research-intensive university. Specific questions that guided this study were: (a) what are the perceived challenges faced by graduate teaching assistants?; (b) what are the sources of help for the graduate teaching assistants?; (c) what are the characteristics of the effective graduate teaching assistant training program? This study was investigated through qualitative methodologies. Data were collected from semi-structured interviews with participants and program and class artifacts. The study was conducted at a doctoral/research-intensive university in the Southeast, with over 10,000 undergraduates. Three participants (1 male, 2 females) who taught activity classes participated in the study. Participants were all U.S. citizens and were in the master's program. Interview transcripts and artifacts were inductively analyzed in order to find emerging themes and categories (Miles & Huberman, 1994). In order to enhance trustworthiness, multiple sources of data and member check interviews (Lincoln & Guba, 1985) were used. Graduate teaching assistants' challenges were categorized into: (a) personal challenges (e.g., balancing teaching assistant duties with their graduate workload, gaining respect from students), and (b) instructional challenges (e.g., lack of student motivation, dealing with student disciplinary problems, dealing with diversity issues in college student population). Participants commonly reported that the support from the program coordinator, discussions with peer teaching assistants, current and previous teaching experience and their graduate coursework helped teaching performance. In order to facilitate the professional development of graduate teaching assistants, participants emphasized the importance of support in light of: (a) graduate teaching assistant training as an ongoing process, (b) discipline-specific training, (c) understanding of diversity of college student population, and (d) establishing a community to share expertise with other teaching assistants and faculty members. Implications of the study were provided in terms of developing effective training programs for graduate teaching assistants and their enhancement of teaching effectiveness.