This article summarizes several case studies examining the successes, challenges, and failures of co-teaching in secondary content classrooms. The key factors found to influence co-teaching outcomes were the academic content knowledge of both teachers, compatibility between co-teachers, and emphasis placed on high-stakes testing. When teachers had distinct roles aligned with their expertise, planned together, and had a respectful relationship, co-teaching was more effective. However, pressure from high-stakes tests often resulted in rapid instruction that hindered individualized support for students with disabilities. Compatibility, use of strategic instruction, and emphasis on student learning versus testing were found to be important for successful co-teaching.