The National Mathematics Advisory Panel, established under the Bush Administration, was created to improve teaching and learning of mathematics in the United States. One component of the study was focused on teachers and professional development opportunities. They found that the majority of professional development studies available were mostly descriptive. Many studies that were considered empirical did not include a comparison group and relied on teachers' self-reports about their knowledge before and after the professional development rather than on measures of teacher knowledge. They concluded that self-reported data cannot continue to be the main source of information about professional development outcomes. Two questions guided this study: "How does a co-learner model in professional development delivery affect teachers' perceived self-efficacy in mathematics instruction?" and "How does a co-learner model in professional development delivery affect teachers' specialized mathematics knowledge for teaching?" This mixed methods study gathered data using a quasi-experimental design that used a non-equivalent control-group design (Gall, Gall & Borg, 2007) supported with qualitative data using end-of course surveys, focus groups, and an interview. The study sought to determine if there was a significant difference in two professional development delivery models to influence teachers' perceived self efficacy on teaching mathematics on three dimensions; student engagement, instructional strategies, and classroom management, and to find if there is an increase in teachers' specialized mathematics conceptual knowledge in additive and multiplicative structures and situational stories classifications. The control group (n=16) received the professional development on site amongst peers and the experimental group (n=15) received the same professional development treatment in a co learner setting. Findings indicate that both groups had significant gains in self-efficacy toward teaching mathematics in student engagement (Group A t=2.6, p=0.023, pre-test M=6.52, post-test M=7.03; Group B t=6.1, p=0.000, pre-test M=6.69, post-test M=7.47), and Group B had significant gains in instructional strategies (t=3.7, p=.003, pre-test M=6.65, post-test M=7.16). Although there were differences between pre-test and post-test for Groups A and B, the ANCOVA indicated that there was not a significant difference between the adjusted post-test group means. There is much potential for future work based on the study's outcomes. Co-learning support systems for teachers could be designed to positively boost perceptions in teaching mathematics as well as other disciplines. Professional development topics could be determined to focus on the specialized content and skills that are unique to teaching a discipline. [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.]