Purpose: Following recent federal legislation and related policy changes, co-teaching evolved rapidly as a strategy to provide students with disabilities access to the same curriculum as students without disabilities while receiving instruction in the least restrictive environment. It is unclear if co-teaching is an effective instructional strategy for educating students with disabilities in the least restrictive environment. The purpose of this causal-comparative study was to determine if there was a significant difference in students with disabilities' academic performance in co-taught versus traditional classrooms. Methodology: This causal-comparative study examined the effects of co-teaching on California students with disabilities' performance on the Smarter Balanced assessments. Data were collected from 3 school districts representing 10 comprehensive high schools; 641 test results from the spring 2016 Smarter Balanced assessments of 11th-grade students with disabilities from co-taught and traditional classrooms in English and mathematics were compared using an independent samples t test. Findings: Analysis of the 641 Smarter Balanced test results produced the following findings: (a) students with disabilities primarily receive instruction in the traditional classroom; (b) in co-taught classrooms, students' primary disability was typically a specific learning disability; (c) t-test results indicated a significant difference in English test scores for students receiving English instruction in co-taught versus traditional classrooms; (d) t-test results indicated no significant difference in mathematics scores for students receiving mathematics instruction in co-taught versus traditional classrooms. Conclusions: There is a statistically significant difference in the academic achievement of students in English. While performing lower than students with disabilities in traditional classrooms, students with disabilities in co-taught classrooms received access to the grade-level curriculum in the least restrictive environment. No statistically significant difference in the academic achievement of students in mathematics was noted, suggesting students with disabilities are performing similarly in mathematics regardless of instructional setting. Recommendations: It is recommended additional research focuses on the academic achievement of students with disabilities in multiple settings, traditional, co-taught, and special education classrooms, to identify potential variations in achievement related to instructional setting. Additional research may determine the instructional setting's impact on students' attitude toward learning, relationships with teachers, or other social-emotional factors. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]