Improving teachers' content knowledge in mathematics is one way to address the ongoing national concern of poor performance in math. The purpose of this study was to investigate the impact of self-regulated online professional development for teachers' knowledge of fractions, decimals, and percentages, as well as factors prompting that impact. Quantitative analysis consisted of pre-test and posttests which showed significant improvements in teachers' content knowledge in fractions, decimals, and percentages. Moreover, teacher's reflections displayed evidence of self-regulation skills both individually and within learning communities. The use of online manipulatives, videos, and collaborative discussion boards enhanced the teachers experience and promoted deeper understanding of fractions, decimals, and percentages. The results of this study indicate that online professional development in fractions, decimals, and percentages increases teachers' content knowledge. [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.]