The purpose of this qualitative phenomenological research study was to investigate the factors that influence mathematical instruction of upper elementary teachers in school districts within Southern California. More specifically, this research study examined teaching strategies of upper elementary teachers' experiences and perceptions regarding implementing student engagement in mathematics in school districts within Southern California. This qualitative phenomenological research study involves teachers who utilize instructional strategies based on their best practices that foster students engaging in upper elementary mathematics. In this research study, the participants explained their lived experiences and perceptions of utilizing their chosen strategies to engage students in mathematics instruction. The literature review referenced in this study exemplifies instructional strategies, which include student engagement in the upper elementary grades that are important to life-long learning. Mathematics was specifically targeted due to the negative perceptions that students have that are often associated with the subject. Students' lack of motivation and low academic achievement are a few of their negative interpretations that are affiliated with mathematics. This study adds to the body of knowledge regarding the factors that influence teachers' instructional decision-making when it pertains to fostering student engagement in mathematics instruction. The study participants included nine upper elementary math teachers, who are working currently in school districts within Southern California. The researcher conducted semi-structured interviews to gather participants' insights and experiences, presenting an opportunity to explore their various perspectives individually. Due to the Covid-19 previous state-mandated restrictions, the examiner conducted the interviews in a Zoom virtual environment. The investigator interviewed nine participants each for one hour with ten open-ended questions that were distributed electronically. There were seven comprehensive themes that emerged during the qualitative analysis process: (1) engaging students in small group collaboration; (2) motivating students through rigorous instruction; (3) utilizing scaffolding techniques; (4) magnifying students' leadership through self-directed activities; (5) implementing high-level cognitive development activities; (6) engaging students in differentiated instruction; and (7) integrating of 21st century technology. The seven themes were congruent to the theoretical framework of constructivism, and the literature review. [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.]