The use of student-centered practices is influenced by several factors (Peterson et al., 1989). Specifically self-efficacy has been shown to influence teachers' self-reported teaching practices (Hadley & Dorward, 2011; Peterson et al., 1989; Skaalvik & Skaalvik, 2007). The purpose of this study was to determine possible relationships among effective teachers' mathematical and mathematics teaching self-efficacy, instructional beliefs, and the enacted use of effective practices in mathematics. The study involved two K-6 mathematics teachers who were identified as effective by recommendations from highly regarded mathematics teacher educators or administrators. To determine teachers' level of self-efficacies, instructional beliefs, and enacted teaching practices, I used self-efficacy surveys, multiple observations, and a stimulated recall end-of-study interview. Using a descriptive multi-case study methodology (Yin, 2003), I examined the relationships among the three factors (i.e., self-efficacy, instructional beliefs, and practices) of my participants. I found that the teachers' mathematical self-efficacy (MSE) influenced their mathematics teaching self-efficacy (MTSE). Additionally, teachers' self-efficacy interacted with their instructional beliefs and enactment of Standards of Mathematical Practices (NGA & CCSSO, 2010) and mathematical teaching practices (NCTM, 2014). Although teaching during a global pandemic was difficult, the teachers were able to adapt in ways that reflected their instructional beliefs and allowed them to enact effective teaching practices. The resiliency of these effective teachers underscores the value of developing and supporting effective mathematics teachers. [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.]