The purpose of the study was to explore a conceptual structure of graduate teaching assistant (GTA) teaching concerns. Results indicated that GTAs experience five distinct, inter-related types of concerns: class control, external evaluation, task, impact and role/time/communication. These "teaching concerns" were further analysed by examining their relationship with the value placed on them by the GTAs and the confidence in dealing with the teaching-related issues of concern. The results revealed that GTAs tend to have concerns about self, task or role/time/communication-related issues when the nature of the concerned issues is perceived as being valuable but challenging. On the other hand, GTAs are more likely to have concerns with impact-related issues when the nature of the issues is perceived as both being valuable and manageable. Several GTA characteristics, such as teaching experience, teacher efficacy, participation in professional development and values on teaching practices, were examined as predictors of GTA teaching concerns. (Contains 2 tables.)