The purpose of this study was to develop and validate items with which to assess A. Bandura's (1997) theorized sources of self-efficacy among middle school mathematics students. Results from Phase 1 (N=1111) were used to develop and refine items for subsequent use. In Phase 2 of the study (N=824), a 39-item, four-factor exploratory model fit best. Items were revised to strengthen psychometric properties. In Phase 3 (N=803), a 24-item, four-factor confirmatory factor model fit best. This final model was invariant across gender and ethnicity. Subscales correlated with self-efficacy, self-concept, mastery goals, and optimism. Results suggest that the sources scale is psychometrically sound and could be adapted for use in other domains. (Contains 1 figure and 6 tables.)