This dissertation investigated regular and special education co-teachers' perceptions of professional development programs and their readiness to co-teach in the elementary mathematics classroom. A gap in co-teaching literature exists regarding how teachers perceived co-teaching professional development and how it impacts their readiness to co-teach. Two central research questions directed the study. Question 1 asks, "What are elementary mathematics co-teachers' perceptions of co-teaching professional development?" Question 2 asks, "How does professional development effect co-teaching in the elementary mathematics classroom?" A basic qualitative study was used to uncover, illuminate, and describe perceptions that mathematics co-teachers hold regarding professional development and readiness to teach in an elementary mathematics setting. Data were gathered in the form of one-on-one interviews with both regular and special education elementary mathematics co-teachers. A sample of 15 co-teachers was drawn from a school district in Pennsylvania where co-teaching was utilized in the elementary mathematics classroom. Interview data were recorded, transcribed, and coded in order to create themes used to answer the research questions. Major themes that arose to answer Research Question 1 included: (a) first exposer to co-teaching, (b) initially helpful, (c) continued professional development is necessary, and (d) professional development focused on co-teaching models. Themes that emerged used to answer Research Question 2 were: (a) professional development increased the use of co-teaching models/strategies, (b) professional development encouraged collaborative practices, and (c) professional development showed the importance of co-planning with professionals. Findings of the study were consistent with current literature and the theoretical framework of the critical reflection and social cognitive learning theories. Implications of the study and recommendations for future research were provided. [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.]