A future instructor's teaching beliefs contribute to whether they will use research-validated teaching strategies. We surveyed and interviewed potential future geoscience faculty as represented by graduate students and post-doctoral scholars to characterize their teaching beliefs, evaluate changes in those beliefs over time, and identify experiences that fostered interest in teaching. This study employed a longitudinal mixed-methods experimental design that began with surveys of more than 600 geoscience graduate students and post-doctoral scholars, continued with interviews of sixty of these participants, and concluded with detailed case studies of ten individuals. Case study interview results indicate that professional development has the largest impact on teaching beliefs. Mentors were also influential but may have had either positive or negative effects on the development of reformed beliefs. Participants' desire to seek a career involving teaching was motivated by their experience as a student, external encouragement, and the potential to have an impact on future students. Our findings indicate that participation in long-term effective professional development, having a relationship with a mentor that views teaching positively, and having opportunities to teach as a PhD student or post-doc contribute to the development of reformed teaching beliefs and foster an interest in teaching as a career.