Mathematics instruction in US schools is largely inconsistent with visions of research and policy, which suggest instruction should be student-centered, maintain high levels of cognitive demand, and support rich mathematical discussion and argumentation. Professional development (PD) figures prominently in addressing this issue, and studies have shown that teachers' involvement in community-based PD is one way to promote instructional change. This study investigated teachers' participation in online community-based PD that was designed to support participants' development of research-based instructional practices and concurrent enculturation into the Math Forum--one of the premier groups for mathematics and mathematics education in the United States. The design of the online PD included activity mediated by the EnCoMPASS Environment, a specifically designed web-based assessment environment that scaffolds participation in particular practices. This study found that as a result of participation in the online PD course participants began to participate in a social norm and sociomathematical norm for developing mathematical explanations that examine quantitative relationships and engage covariational analysis. Moreover, the EnCoMPASS Environment scaffolded participation in an evidence-based feedback practice and participation in this feedback practice transferred across modalities in the course. This study has implications for supporting teachers' instructional change and contributes to an understanding of a scalable design for mathematics teacher professional development that can support teachers' development of generative and productive norms and practices as well as position teachers for more prolonged engagement in online community of 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.]