This dissertation comprises three projects that were designed to investigate the extent that mastery experiences, verbal persuasions, vicarious experiences, and physiological state contribute to instrumental music teachers' self-efficacy. The first investigation was a review of the literature about self-efficacy and its sources among general education teachers, music teachers, and preservice teachers. The second investigation was a survey study of how instrumental teachers' self-efficacy for teaching strategies is influenced by its four sources. Results indicated concert band directors' self-efficacy related the most with mastery experiences, followed by verbal persuasions, physiological state, and vicarious experiences, respectively. Further investigation indicated levels of self-efficacy related with years of experience and self-perceptions of effective teaching. The third investigation was a case study of preservice instrumental teachers' self-efficacy and concerns. Participants' beliefs were most impacted by their familiarity with a setting and perceived success of previous pedagogical experiences. Results from these three projects indicated that instrumental music teachers' self-efficacy (a) is most influenced by the relative success of previous teaching experiences, (b) relates to effective teaching behaviors, and (c) improves with experience and familiarity in a setting.