Co-teaching, in which a general and special educator collaboratively teach a class comprised of students with and without disabilities, is a common service delivery model school districts employ to increase inclusion of students with disabilities (SWD) in general education.Co-teaching is intended to provide access to the general curriculum for SWD and to provide the specialized instruction they need. However, despite the widespread use of co-teaching in schools and the high cost of placing two teachers in one classroom, there is still no clear, agreed-upon definition in the literature for what co-teaching should look like in practice (what "specialized instruction" means, for example), nor is there evidence of its effectiveness in improving students' academic outcomes.Using joint outcome production theory as a foundation for the conceptual framework, this study adds to the research base on co-teaching by closely examining specialized instruction in co-taught high school classrooms. According to this theory, as SWD are included in general education classrooms, the demand on teachers' resources increases. Adding a second teacher to the classroom should address that increase and result in a gain in resources that should benefit all students in the class. The question becomes how to best deploy those additional resourceswhether the two teachers should be filling different roles in the classroom or doubling up on existing roles. This study employed a concurrent mixed methods design to answer the following research questions: a) What does instruction look like in co-taught high school classrooms? b)What school-level structural factors are in place surrounding co-teaching? and c) What teacher-, classroom-, and school-level factors are related to observed patterns of instruction? To answer these questions, I conducted observations in two rural schools across 10 co-taught classrooms and in corresponding solo-taught classrooms to identify specific examples of specialization. In addition, participating teachers and three administrators were interviewed to determine their approaches to co-teaching and their experiences with the ways in which it is implemented in their schools. Teachers also completed a survey about their experiences with co-teaching and the details of their daily schedules.I conducted two or three observations of each individual class using an observational tool called the Classroom Teaching Scan across all 10 teams for a total of 60 observations and 64 hours. Statistics were calculated regarding time use in each classroom, rates of questions and feedback, and amount of time spent in specific instructional practices. Data were also collected and analyzed on the implementation of key instructional practices, in order to compare the methods used to those recommended by the research with regard to effective instruction for SWD. I compared these data across settings and teams to create detailed descriptions of the instruction provided in co-taught classes and its adherence to evidence-based practices. I...