Originally founded as a private means for diplomats' children to earn an internationally recognized high school diploma, today the International Baccalaureate (IB) Diploma Programme (DP) serves students from a variety of backgrounds in 144 countries. The IB mission and Learner Profile--consisting of ten attributes to foster in students--suggest that development of students' citizenship is the core purpose of an IB education. In this study, in four public schools in California that offer the DP, I interviewed DP Coordinators, teachers (n = 15) and students (n = 24) to learn their perspectives about how and the extent to which the DP develops students' citizenship. I also surveyed students using items that were administered as part of other surveys to large samples of high school-enrolled 17 and 18 year olds. The results indicate that the DP's heavy pedagogical reliance on discussions, debates, oral presentations, written assignments and teamwork, enables students to develop many of the skills that are necessary for civic advocacy. Results also indicate that the DP places a strong emphasis on students' knowledge of issues related to public policy. Interview results indicate that the DP does not seem to strongly prioritize students' knowledge of U.S. government structure and functioning. The student survey results, however, show that compared to nationally representative samples of similarly aged 12th-grade students, the sample of IB students scored higher on nine of ten items that tested their knowledge of U.S. government history and functioning. The DP develops students' citizenship through promoting their awareness of political and social issues and required active engagement with a local or global issue. Most IB students and teachers feel that the DP develops students' academic civic mindedness and model citizenship to a considerably greater extent than other curricular alternatives (including California College Prep and Advanced Placement courses). Finally, teachers feel that the strongest limitation to their prioritization of students' citizenship development is their lack of clarity on how to frame the civic implications of the knowledge, skills and attitudes they seek to develop. The paper includes the following appendices: (1) Student interview protocol; (2) Teacher interview protocol; (3) Student survey; and (4) Student survey details.