The population of U.S. schools has shifted dramatically in the past two decades to include many more linguistically and culturally diverse learners, while the teacher population has remained largely White and monolingual, with limited connections to immigrant communities. Among the many changes diverse learners have brought to U.S. schools is the increased need for the teaching force to understand how to teach English language learners (ELLs) effectively. One solution to supporting ELLs has been an increase in English to speakers of other languages (ESOL) specialists "plugging in" to grade-level mainstream classrooms so that they can benefit from interactions with English-dominant peers as well as content instruction in English. The inclusion of ELLs and ESOL specialists in mainstream classrooms is a relatively new phenomenon, and many researchers, policymakers, and practitioners are interested in how collaborating teachers learn and work in a variety of settings. The goal of this qualitative study was to explore teacher learning through the co-construction of specialized knowledge and practices between ESOL specialists and mainstream teachers as they collaboratively planned, taught, and reflected on lessons. The data emerged from the second year of a 3-year federally funded cross-age peer tutoring (CAPT) reading intervention designed to support vocabulary development and reading comprehension of ELL kindergartners and fourth graders who worked in little buddy-big buddy pairs to read researcher-created texts with science, technology, engineering, and mathematics themes. The CAPT program was taught during the regular English language arts (ELA) block and was supplementary to the students' regular ELA curricula. The research team developed eight text-based lesson sets, which included a mixture of narrative and expository texts centered around the themes of caring for the environment and measurement. One of the positive aspects of teachers' experiences in this study is that teachers with different specializations, strengths, orientations, and background knowledge participated together in the same instructional event, experienced student learning within the space of that shared occurrence, and reflected together on what students struggled with--and what they learned. This gave teachers a common set of experiences around which to build a shared understanding of how to support students, thus setting groundwork for shaping the teachers' thinking, their practices, and their students' opportunities for greater academic success. This study serves as an important foundation for future work exploring how to support teachers and students as they participate in a new era of reform-based instruction and learning.