Mathematical cognitive technology tools, such as Desmos, have the capacity to provide rich mathematical experiences for students (Bos, 2009; Dick & Hollebrands, 2011). To maximize the benefits of MCTs, teachers must believe the technology will help students understand mathematics and be skilled at using the technology (Doerr & Zangor, 2000; Pierce & Stacey, 2010). Thus, the technology can be used to create student-centered learning environments (Drijvers, 2015). The purpose of this research was to study the impact of PD designed using the Pedagogical Technology Framework (Thomas & Hong, 2005) on teachers' orientations about and instrumental genesis in using Desmos in instruction. This study indicated a cyclic relationship between the teachers' orientations and their instrumental genesis in implementing Desmos. As the teachers' technological knowledge (TK) in implementing Desmos increased, their confidence in using the technology also grew. Reciprocally, as their confidence grew, their TK expanded to use increasingly sophisticated Desmos features. Their confidence and TK nurtured their beliefs and the evolution of their instrumental genesis. The teachers believed that using Desmos informed their instruction, thus they used Desmos's features to create learning experiences for their students. The teachers also believed using Desmos created opportunities for them to be facilitators of learning. Correspondingly, the teachers utilized Desmos tools to create student-centered environments. As teachers gained more experience, they focused on how Desmos impacted their pedagogical decisions in supporting student learning. The critical junctures in the evolution of the teachers' orientations and instrumental genesis occurred when they implemented Desmos lessons with their students. [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.]