Background/Context: Billions of dollars are spent annually on professional development (PD) for educators, yet few randomized controlled trials (RCT) have demonstrated the ultimate impact PD has on student learning. Further, while policymakers and others speak to the role schools should play in developing students' civic awareness, RCTs of PD designed to foster civic learning are rare. This randomized controlled trial contributes to the knowledge base on the effectiveness of PD designed to integrate civic learning, ethical reflection, and historical thinking skills into high school humanities courses. Focus of Study: The study examined the impact of a PD intervention in two areas: (a) teacher self-efficacy, burnout, and professional engagement and satisfaction; and (b) the academic, civic, social, and ethical competencies of 9th and 10th grade students in the teachers' classes. Population/Participants/Subjects: The study involved 113 teachers and 1,371 9th and 10th grade students in 60 high schools from eight metropolitan regions in the United States. Intervention/Program/Practice: The intervention, Facing History and Ourselves, provides PD through a five-day seminar, curricular materials, and follow-up coaching and workshops to help teachers develop their capacities to implement an interdisciplinary historical case study unit using student-centered pedagogy. Research Design: The study used a school-level, randomized, experimental design to investigate impacts of the intervention for teachers and their 9th and 10th grade students. Findings/Results: Intervention teachers showed significantly greater self-efficacy in all eight assessed domains, more positive perceptions of professional support, satisfaction and growth, and greater personal accomplishment. Intervention students demonstrated stronger skills for analyzing evidence, agency, and cause and effect on an historical understanding performance measure; greater self-reported civic efficacy and tolerance for others with different views; and more positive perceptions of the classroom climate and the opportunities afforded for engaging with civic matters. Fidelity analysis found these causal effects despite the fact that roughly half of the intervention teachers did not fully implement the program. Conclusions/Recommendations: Educators need evidence-based approaches for teaching complex social, civic, and political issues enabling students of diverse mindsets and backgrounds to engage constructively with one another while obtaining necessary skills and knowledge. These findings provide empirical support for a professional development approach that engages teachers in fostering academic and civic competencies critical to both participation in a democracy and success in college and career.