Co-teaching is a method that is increasing within schools across the US as educators strive to leave no child behind. It is a costly method, having two paid instructors in one classroom, with an average of 24 students shared between them. If it significantly increases the achievement of all students, it is well worth the costs involved. However, few studies have analyzed the effectiveness of this method on student achievement. This research follows the academic accomplishments of students in a ninth grade physical science course. Nine sections of the course "Force and Motion" were taught with a single teacher, and two additional sections were co-taught, one led by a science-certified and special educator, and another co-taught by two science certified teachers. Subgroup achievement performance was analyzed to determine whether significant differences exist between students with or without IEPs, as well as other factors such as free and reduced lunch status or gender. The results show significance with the presence of a co-teacher, while there is minimal effect size of co-teaching in this study for students with IEPs. The benefactors in these ninth grade co-taught classes were the students without IEPs, an unintended result of co-teaching. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]