This study used a constructivism theoretical framework to focus on a group of elementary mathematics teachers and their lived experiences after the introduction of new mathematical content standards through the lens of the Mathematical Teacher's Specialized Knowledge Model. After the introduction of new mathematics content standards, many elementary teachers were asked to reevaluate their understanding of how they taught multiplication of whole numbers. The study focused on the experiences of six elementary teachers and any changes that occurred to their approach, understanding, and implementation of teaching the concept of multiplication of whole numbers based on the introduction of new mathematics content standards. The research implemented a phenomenological qualitative design paradigm built around teacher's lived experiences after the 2014 implementation of the updated mathematics Texas Essential Knowledge and Skills content standards. An interpretive phenomenological analysis approach was used to understand the teachers' lived experiences in order to describe the process of adapting teaching practices to the demands of a new set of content standards. The data were reported with respect to the pedagogical content knowledge sub-domains of MTSK. Teachers reported making significant changes to their approach to teaching multiplication of whole numbers, which included an emphasis on pedagogical strategies to encourage the development of conceptual understanding. Teachers made changes to the way they planned their lessons to incorporate more pedagogical strategies to help develop students' conceptual understanding of multiplication of whole numbers. Teachers continued to adjust their pedagogical strategies based on the needs of the students within their classroom. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]