This study explores the impact of providing dedicated time and professional learning focused on cross-curricular lesson planning on teacher efficacy and pedagogy. Grounded in Bandura's theory of Collective Teacher Efficacy (CTE) and guided by Root-Bernstein and Root-Bernstein's concepts of interdisciplinary problem-solving, this research aimed to assess how cross-curricular professional learning affects teacher collaboration, creativity, and instructional practices. This study was a concurrent mixed-methods action research study conducted at a middle school in a school district in the Southeastern United States. Participants engaged in a four-month long professional learning experience focused on Lesson Study as a framework for PL to design cross-curricular lesson plans. The findings indicate that while the professional learning initiative led to improvements in teachers' understanding of collective efficacy and collaboration, these changes were not statistically significant. However, qualitative analysis revealed a positive shift toward student-centered teaching practices, increased cross-curricular collaboration, and creative lesson planning. Teachers also expressed concerns about the sustainability of these changes without ongoing support. The study concludes that sustained professional learning, administrative support, and interdisciplinary collaboration are key factors in fostering long-term changes in teacher efficacy and pedagogy. Future research should focus on longitudinal studies and comparative analyses of professional learning models to determine which approaches are most effective in enhancing teacher collaboration and instructional practices. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]