The purpose of this study was to investigate the four sources of self-efficacy in music performance and examine responses from the Music Performance Self-Efficacy Scale (MPSES). Participants (N = 290) were middle and high school music students from 10 schools in two regions of the United States. Questions included the following: (1) How much influence does each source have on self-efficacy? (2) Are there differences among band, chorus, and string students? (3) Are there differences between middle and high school students? (4) Does music aptitude predict self-efficacy, and (5) Does evidence support the MPSES as a valid and reliable scale? Results indicated that mastery experience exerted the strongest influence, no differences were found among ensemble types or grade levels, and music aptitude scores predicted modest increases in self-efficacy, ß = 0.16 (0.07). Examination of test content, response process, internal structure, and relationships with other variables provided evidence of validity, while internal consistency and test-retest values provided evidence of reliability. Recommendations for further research included examining the development of self-efficacy at different ages, measuring the relationships between the sources of self-efficacy and music achievement, and using the MPSES to assist in the investigations of other psychological constructs, such as self-identity and motivation.