Effective and authentic communities of practice in schools have the potential to support teachers in improving their instructional practices around perennial challenges, such as improving the literacy skills of all students. But before they can achieve such goals, communities of practice take time to build, effort to sustain, and ongoing support to spread their work. Because a strong community of practice is often situated within a broader department or school context, an ecosystem within an ecosystem, nurturing that community requires a delicate balance of supports and structures if it is going to lead to real instructional change. The authors work in an ongoing disciplinary literacy professional learning initiative has taught them that the formation of communities of practice for teachers relies on finding the right balance of elements that both support such communities and also free teachers to pursue authentic work related to their own classrooms. While this just-right balance is often built through trial and error, and necessarily changes over time, it is an essential element of a productive community of practice. Moreover, they believe that there are several broad tensions that could be instructive to new communities of practice as they design their own professional learning trajectories. These communities of practice were formed as part of the Content-area Reading Initiative at Brookline High School in Brookline, Massachusetts, a large and diverse comprehensive high school. Brookline High School has more than 140 teachers, who serve over 1,700 students representing 76 nations and speaking 57 languages. Roughly a third of students are English language learners, and a growing number of students receive free or reduced lunch or special education services. The Content-area Reading Initiative, designed partly in response to shifting student demographics, is a four-year project using teacher professional learning communities to improve students' literacy skills in various secondary content areas. The initiative relied on a variety of structural supports and components to form and support departmental and cross-departmental communities of practice focused on literacy teaching and learning. For the teacher teams involved, finding the right balance between complex factors in the broader school and modifying traditional ways of engaging in professional development made all the difference in spurring changes in teacher practice and student learning. Yet arriving at those changes was not easy or straightforward. This article presents some of the key tensions that emerged throughout the project and that members of communities of practice navigated to work and learn together effectively. While the authors caution that not all communities of practice will encounter these same tensions, they believe that considering the various factors that shaped particular communities of practice work within a particular ecosystem can help others consider the tensions that might arise in their context.