Reform standards in mathematics education have called for classrooms that are student-centered and that incorporate problem solving and reasoning for meaningful learning. After decades of reform efforts involving multiple stakeholders, research indicates that most classrooms remain teacher-centered and procedurally focused, due to the complexity of concerns and competing intentions that teachers face in their work. Coteaching, a commitment between two teachers to coplan, coenact, and coreflect on lessons, can serve as an ongoing, sustained, focused, integrated, reflective professional development structure that supports teachers towards growth. The theoretical constructs used to describe possible growth towards reform orientations in teaching secondary mathematics include craft knowledge gained in practice, shifts in actions of the teaching habitus, and zones of proximal development among coteachers. This study questions how coteachers can serve as resources for each other's professional growth towards reform-oriented pedagogy. Through a qualitative, naturalistic inquiry, a case study methodology was used to focus on one pair of coteachers in a year-long study of their high school Algebra 1 coteaching. The analytic framework includes the stages of development of forming, norming, storming, and performing, as well as the process, content, and levels of reflection. Findings regarding teachers' actions as resources towards reform-oriented teaching include modeling and observing teaching, sharing ideas for teaching, communicating with and challenging each other, and creating alternatives for teaching. These actions, along with reflection, demonstrate possibilities for transitioning teaching practices towards interactive, intellectually engaging pedagogy through purposeful collaboration and deep reflection on, and in, practice. [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.]