In elementary mathematics instruction, teachers often compromise access and cognitive demand in pursuit of one or the other. I explored how five different co-teaching models impact students' access to cognitively demanding mathematical learning opportunities (MLOs) in elementary classrooms. Through ethnographic lesson observations, artifact analysis, and teacher reflection, I investigated how one teacher pair used five different co-teaching models during mathematics instruction in a second-grade classroom. The accessibility and cognitive demand of MLOs during the enactment of each co-teaching model were analyzed to explore the affordances of each co-teaching model. The Universal Design for Learning Math (UDL Math) framework was applied as a measure of student access to MLOs across the co-teaching models, and the Instructional Quality Assessment for Academic Rigor (IQA-AR) was used to analyze the cognitive demand of MLOs. Qualitative analysis of video-recorded lessons, documented artifacts, recorded interviews with teachers, and written survey responses revealed Teaming to be a powerful co-teaching model for instruction that is accessible and maintains cognitive demand. All five models revealed affordances for different elements of accessibility during cognitively demanding instruction. These findings support teachers to design and enact co-taught mathematics instruction more intentionally to meet all students' needs and provide guidance for teacher educators to prepare pre-service teachers for 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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]