Purpose: The purpose of this single-case study was to explore principals' perceptions of self-efficacy beliefs as effective instructional leaders during a period of educational transition in a semiurban, unified school district in Southern California. Methodology: The researcher used exploratory case study, conducting semistructured, open-ended, interviews in private settings, eliciting principals' self-efficacy perceptions. The researcher interviewed eight principals, elementary through high school, using a social constructivist interpretive framework. Findings: The theoretical framework was Bandura's theories of agency, efficacy, and alignment to The Wallace Foundation's research of effective leadership practices. The following eight broad areas indicate how principals' self-efficacy impacts student achievement and how environment influences principals' self-efficacy: This is significant change, having a process will help, collaborate to get the best ideas, data informs and has many formats, everything is new, principals need support too, principals maintain a vision, and determining meaningful feedback. Conclusions: This study led to recommendations supporting principal efficacy and aligning to The Wallace Foundation's research on effective leadership practices, revealing the need for improving data-informed decision making, defining evidence-based classroom practices with monitoring and support, establishing external-internal teams to build leadership around effective practices, creating intradistrict principal networks fostering collaboration and growth, and developing multisource feedback instruments for evaluation and leadership development. Recommendations: Principal efficacy remains important based on the conclusions. Future research should explore structured principal learning networks' impact on efficacy, relationships between new accountability models and principals' self-efficacy, longitudinal impact on professional standards for educational leaders on efficacy, and relationships between efficacy and multisource evaluative feedback assessments. [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/en-US/products/dissertations/individuals.shtml.]