The purpose of this study was to examine the influences of various facets of middle school mathematics teachers' practical rationality on their instructional decision making as they plan to enact the Common Core State Standards for Mathematical Practice, CCSS-MP1 (perseverance in problem solving) and CCSS-MP3 (communicating and critiquing). The study expands and confronts some of the literature that lacks specifics to instructional decisions made by middle school mathematics teachers, particularly with regards to their disposition, the operationalization of their MKT, and the connection to the institutional obligations affecting their practical rationality. This study provides insight into three facets impacting middle school mathematics teachers' instructional decision making with regards to CCSS-MP1 and CCSS-MP3. First with regards to teacher disposition, a facilitative role using a problem solving approach is preferred. Secondly, curricular choices are made directly tied to the teachers' MKT. Finally, although the teachers recognized that they had certain obligations to the institution, they also held the belief that the institution was obligated to them as well. The two main obligations required from the institution were the need for professional development provided by the district and appropriate resources that promote problem solving and communication. These results suggest the need for professional development for in-service and preservice teachers in the facilitative approach to mathematics instructions, more time for teachers to work collaboratively in professional learning communities (PLCs), professional development for district administrators, board members and parents with regards to a more rigorous and investigative approach to mathematics instruction. [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.]