With the recent emergence of online professional development (PD), such opportunities are increasingly available to educators. It is important, then, to assess whether and how such options can be valuable and effective for them. The goal of this study was to assess whether and how a Massive Open Online Course for Educators (MOOC-Ed) can serve as an effective PD option for educators by focusing on how and to what extent a MOOC-Ed on "Fraction Foundations" changed participants' knowledge, beliefs, and practice. In order to address the goals of the study, a convergent, parallel mixed methods design was used in which both quantitative and qualitative data were analyzed to inform a case study on the MOOC-Ed. Data were collected through a number of instruments administered to all participants in the MOOC-Ed. Pre- and post-surveys addressed participants' attitudes and beliefs about the effectiveness of different components of the PD program, and a pre- and post-assessment addressed participants' mathematical knowledge for teaching (MKT). The course on "Fraction Foundations" was administered twice, once in the Fall and once in the Spring. Fifty-four participants submitted all surveys and assessments in the first course, and 159 participants submitted them in the second. Respondents primarily included teachers (especially of elementary school students), teacher leaders, administrators, and curriculum and instruction specialists. Additional qualitative data were collected through personal interviews and classroom observations of four elementary teachers to contextualize the interconnections between changes in participants' knowledge, beliefs, and practice. The "Interconnected Model of Professional Growth" (Clarke & Hollingsworth, 2002) was used as a framework to analyze results from these qualitative data and develop narratives to describe the teachers' movement through their "growth cycles". Results from this study showed that a MOOC-Ed can serve as an effective form of professional development in changing participants' knowledge and/or beliefs and having an impact on their practice in a number of ways. Design principles implemented in the development of the MOOC-Ed, which included 1) multiple voices; 2) self-directed learning; 3) peer-supported learning; and 4) job-connected learning, contributed to participants' perceptions of specific changes in their knowledge and practice. Results also showed that there is a dynamic interplay between participants' perceptions of changes in their knowledge and/or skills as a result of their participation and the impact the PD has on their 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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]