Teacher professional communities are central in the current discussion of teacher learning and professional development. Instructional reform initiatives aimed at improving student achievement through changes in instruction depend on effective professional development structures. Increasingly, teacher professional communities are being identified as central locations for teacher learning through collaboration and instructional changes. Therefore, many leaders are turning to structures that build communities within their organizations, but researchers are debating whether and how leaders can influence collaboration within professional communities. To promote collaboration that aligns with instructional initiatives, some leaders have attempted to design communities, mandating that teachers regularly meet during workdays. Little is known about whether designed communities actually bolster district goals or how, if at all, they facilitate collaboration. Further, little is known about what external conditions may influence how these communities function. This qualitative comparative case study examined two designed communities of middle school teachers within one district reform initiative. The findings in this study illustrate a contrast in how the two professional communities functioned. One designed community perceived similarities in their practices and engaged with specificity, developing a repertoire directly relevant to their classroom practices. This designed community acted as an effective lever for the implementation of specific practices that aligned with district goals, while the second community did not. A variation in external resources appeared to influence the differences in how groups functioned. Specifically, one group's access to experts with deep and congruent conversations around instruction directly impacted how this community functioned. [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.]