Federal law mandates that students with disabilities (SWDs) receive specially designed instruction (SDI), which includes the adaptation of the content, methodology, or delivery of instruction to meet SWDs’ unique needs, to ensure access to the general education curriculum (Rodgers et al., 2021; Ten Napel, 2017) within the least restrictive environment (Individuals with Disabilities Education Act, 2004). One common service delivery model for the many SWDs in the general setting education is co-teaching, wherein a content-area teacher (CAT) and a special education teacher (SET) share instructional responsibilities. The CAT and SET can use a variety of models (e.g., team teaching) to implement co-teaching. Although research showing the effectiveness of co-teaching for improving student achievement is limited (Clancy & Wexler, see Chapter 2; Murawski & Swanson, 2001), co-teaching has the potential to increase student engagement due to the defining features of certain co-teaching models that may benefit SWDs. Increasing student engagement is important as engagement is positively correlated with student outcomes, such as retaining information, graduating from high school, and pursuing postsecondary education (Finn, 1993).To better understand the extent to which different co-teaching models are implemented and which teacher (i.e., CAT or SET) leads instruction during the implementation of certain co-teaching models, it is necessary to extend previous research (e.g., Wexler et al., 2018). Additionally, given the importance of engagement and the potential relationship between co-teaching and engagement, it is necessary to explore whether specific co-teaching models are associated with higher levels of student engagement. Thus, there are two goals of the current dissertation. The first goal is to investigate the frequency of use of each co-teaching model and the extent to which each co-teacher leads instruction during the implementation of certain models. The second goal is to explore the relationship between each observed co-teaching model and student engagement. The current manuscript includes a statement of the problem, theoretical framework, literature synthesis, research questions, methodological approach, results, and discussion for the study. I provide this information sequentially over five chapters. Chapter 1 introduces the problem the current dissertation seeks to address. This chapter provides an overview of current service delivery models for SWDs in the general education setting, including co-teaching, and includes extended descriptions of each of the six co-teaching models. Chapter 1 also provides an overview of the research on student engagement. The chapter closes by providing a statement of the problem and the theoretical framework. Chapter 2 of the dissertation presents a literature synthesis of experimental studies investigating the effect of co-teaching on student achievement. The purpose of the synthesis is to extend a previous synthesis (Murawski & Swanson, 2001) and provide updated knowledge on the impact co-teaching has on student outcomes. While co-teaching has been a commonly used service delivery model, information about its effectiveness is limited. This synthesis contributes a new understanding of co-teaching as more than 20 years have passed since Murawski and Swanson’s initial synthesis. In Chapter 3, I describe the methodological approach of the empirical study. I used archival observation data to determine which co-teaching models were used most often and which teacher led instructional delivery for specific models (i.e., one-teach-one observe, one teach-one assist, one teach-one monitor). I then investigated the relationships between student engagement and the observed co-teaching models. Chapter 4 provides the results of the empirical study. Results from the observation data showed that team teaching and one teach-one assist were the most relied upon co-teaching models. Additionally, the CAT typically led instruction during implementation of one teach-one assist and other independently driven models. Furthermore, there was a moderate significant relationship between engagement and the co-teaching models. Then, Chapter 5 contextualizes the findings within similar research and the theoretical framework. The findings of the first research question on observed co-teaching models align with similar recent research. The investigation into the relationship between co-teaching models and student engagement aligned with the theoretical framework. Specifically, student engagement was observed more frequently in models where both teachers drove instruction (i.e., alternative, station, and team teaching). In closing, I provide implications for practice as well as recommendations for additional research and present the conclusion.