Teacher collaboration holds great promise for improving educational outcomes. Education policies commonly seek to foster teacher collaboration to bring about improvements in teacher learning, instructional quality, and improved student outcomes. Collaborative approaches including professional learning communities (PLCs), grade level teams, instructional coaching, and co-teaching are common in the reform landscape. Teachers' collaboration is deeply enabled or constrained by school organization and the broader educational systems in which it is embedded. My dissertation studies employ an organizational and systems lens for exploring the embeddedness of teachers' collaboration for policy implementation. Specifically, my studies explore two different policy contexts that utilize collaborative approaches for improvement: the inclusion of students with disabilities and ambitious mathematics instruction. Policies related to inclusion count on teachers' collaboration as a means for ensuring that students with disabilities are appropriately supported in general education settings. With regard to math instruction, schools and districts commonly employ collaborative approaches to enhance teacher professional learning and ultimately enhance the quality of their math instruction. Findings from these studies suggest that using an organizational lens and exploring teachers' practice in terms of collaboration can reveal important barriers and unintended consequences to collaboration for policy implementation. Together, my dissertation studies illuminate the organizational and social aspects of teacher collaborations for policy implementation that matter if collaboration is to be productive for supporting students in inclusive settings or improving teachers' professional learning for math instruction. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]