Background: Teachers describe their colleagues as one of their most valuable resources (Drury & Baer, 2011; Johnston & Tsai, 2018). Teachers often report seeking out colleagues who teach the same grade or subject to get advice or to plan instruction together (Frank, 2009; Spillane et al., 2015). Given that research shows that professional learning opportunities are most fruitful when they are content-specific, job-embedded, and collaborative (Darling-Hammond et al, 2017; Desimone, 2011), many school improvement efforts seek to create professional communities of teachers who teach the same content (e.g., Vescio et al., 2008). However, many teachers face a structural barrier to meaningful professional community when they are the only one in their school, or even district, with a specific teaching assignment. Teachers in rural schools or small districts are particularly likely to face this barrier. Our study defines, identifies, and closely examines the experiences of instructionally isolated teachers in Tennessee. We define these teachers as being the only teacher in their school or in their district in a specific teaching assignment (e.g., 4th grade English Language Arts, 7th grade science, Algebra II, visual arts). We posit that this isolation has implications for the professional experiences and development of teachers. Objectives: This explanatory sequential mixed methods study (Creswell & Plano Clark, 2017) integrates both quantitative and qualitative analyses to better understand the experiences of instructionally isolated teachers. We address the following research questions: (1) How do teachers' professional experiences and career outcomes differ across levels of instructional isolation? and (2) To what extent does instructional isolation account for differences in teacher experiences and outcomes across geographic contexts? Research Design and Data: Our research design first uses administrative and survey data to quantitively examine differences in teachers' professional experiences and career outcomes based on levels of instructional isolation. We then use interview data to further explore and explain these patterns. In the sections below, we briefly describe the sample, data collection, and analytic methods for each component of our study. Administrative and survey data. Sample and data collection: The analytic samples for the quantitative analyses are drawn from statewide administrative records, available through a collaboration with the Tennessee Department of Education. These records capture all public K-12 full-time classroom teachers in Tennessee. We also use data from an annual statewide educator survey. Measures: We measure instructional isolation using annual files linking teachers to courses in specific schools. This allows us to identify teachers who are the only ones teaching specific course(s) in their school or district. Outcome measures include teacher observation scores, value-added scores, attrition, and survey responses capturing teachers' perceptions of their school climate, satisfaction, and experiences with collaboration and professional learning. Analytic methods: We use a series of fixed-effects regression models to estimate differences in each outcome between instructionally isolated teachers and their non-isolated peers. Our models include course fixed effects or school fixed effects to understand the differences across isolation among teachers teaching the same courses or teachers within the same school. We also explore whether isolation mediates differences in professional outcomes for teachers across geographic contexts. Educator interview data. Sample and data collection: The analytic sample for the qualitative analyses includes teachers and leaders from 10 Tennessee schools. We used purposeful sampling to identify districts across two distinct contexts with high levels of instructional isolation in the quantitative analysis: (1) very small districts with one school per level and (2) mid-size rural districts. We also recruited a larger, more urban district to serve as a contrasting case. In all district types, we recruited district staff, principals, and teachers to participate in virtual interviews during the 2021-22 academic year. In schools in which all teachers are instructionally isolated, we recruited teachers across a range of teaching assignments. In schools with varying levels of isolation, we recruited teachers who were isolated and those who were not. Analytic methods: We conducted semistructured interviews with all participants. Interviews with leaders focused on how they support teachers and decision-making about teaching assignments. Teacher interviews focused on their career decisions, professional development, and peer collaboration. All interviews were audio recorded and then transcribed. For data analysis, we used an iterative approach with both inductive and deductive thematic coding (Strauss & Corbin, 1990). We constructed comparative cases to illustrate similarities and differences across different types of isolation (e.g., elementary teachers who are isolated by departmentalization, secondary teachers in schools in which everyone is isolated). Findings: The initial analyses indicate that about one-fifth of Tennessee teachers are instructionally isolated, and that isolation is concentrated in rural areas, in small districts, and in certain teaching assignments (see Figure 1). Our quantitative findings suggest that certain career outcomes vary across levels of instructional isolation. For example, isolated teachers report significantly less frequent collaboration than teachers in their same school who are not isolated (see Table 1). However, isolated teachers report similar levels of satisfaction and have lower attrition rates than their non-isolated peers. Our qualitative findings highlight experiences common to instructionally isolated teachers. For example, isolated teachers rarely report ongoing collaborative work with their colleagues around instruction, and many describe short-term or one-way sources of instructional ideas (e.g., meeting with a peer in another district once per year or seeking out instructional ideas on social media). However, isolated teachers also identified specific benefits of isolation, including increased instructional autonomy and less mandated collaborative activities. Conclusion: This analysis focuses on an understudied but important feature of teachers' work in certain contexts: instructional isolation. Isolated teachers often do not have colleagues in their immediate vicinity with whom they can collaborate specifically around instruction. Both our quantitative analyses illustrating the prevalence of isolation and qualitative findings in which educators themselves describe the experience underscore that instructional isolation is a feature of the organizational structure of schools and districts that can have professional consequences for teachers. Our analyses highlight both positive and negative consequences of isolation as well as school, district, and regional efforts meant to better support isolated teachers.