The mindsets of students in upper-level or honors mathematics courses can impact their success, as can their teachers' mindsets. The purpose of this study was to examine students' perceptions of ability and intelligence (mindset) and their perceptions of their teachers' mindset in honors or upper-level high school mathematics classroom to identify changes in students' mindset over time. This sequential mixed-methods study was conducted in a mid-sized International Baccalaureate high school. Mindset surveys were completed by four teachers and 18 students, with the students taking the survey both pre- and post-study. Observations of the teachers provided information on their mindset practices while student interviews offered insight into the students' perception of the teachers' use of mindset practices in the classroom. The changes in students' mindset, along with differences between gifted and non-gifted students' mindset, were analyzed. The results indicated that there was a difference between gifted and non-gifted students' mindsets, there was close alignment between the teachers' mindset and their classroom practices, and that there was some correlation between students' change in mindset and students' perception of teachers' mindset. The results suggest the use of open-ended collaborative tasks and explorations were beneficial to the mindsets of gifted and non-gifted students in high school mathematics classes. Both the growth mindset and mathematical mindset theories are built upon deliberative theory, pragmatic theory, and social cognitive theory. This study found evidence of all these theories in both the quantitative and qualitative data and results. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]