Low mathematics achievement in the United States has led to a push to increase the quality of mathematics instruction. Policy efforts in mathematics have typically focused on increasing teachers' mathematical content knowledge (MCK), with the goal of increasing teacher quality, and in turn increasing student mathematics learning. While research indicates that teacher MCK is predictive of student mathematics achievement gains (Hill et al., 2005), mathematics PD programs focused on increasing teacher MCK have been largely ineffective at increasing student mathematics achievement (e.g., Garet et al, 2016). An alternative approach to increasing student mathematics achievement is to investigate curricula that can be effectively used by teachers with a range of MCK (Agodini & Harris; Stein & Kaufman, 2010). The current study contributed to this line of research using data collected for the Early Learning in Mathematics (ELM) large-scale efficacy trial (Clarke et al., 2015) to investigate two research questions: (1) Does curriculum (ELM vs. business-as-usual) moderate the association between teacher MCK and instructional behaviors?, and (2) Does curriculum (ELM vs. business-as-usual) moderate the association between teacher MCK and student mathematics achievement gains in kindergarten? Participants included kindergarten students (n = 2,598) and their teachers (n = 130) in classrooms randomly assigned to use the ELM core kindergarten curriculum or business-as-usual, district-approved curricula. Multiple regression analyses demonstrated that the nature of the Teacher MCK x Curriculum interaction varied across instructional behaviors examined. Two-level hierarchical linear models revealed that there was not a significant Teacher MCK x Curriculum interaction on student mathematics achievement gains, and main effects models indicated a small but negative effect of teacher MCK and a positive effect of ELM. Results are discussed in the context of implications for future research and practice. [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/en-US/products/dissertations/individuals.shtml.]