Policy efforts in mathematics have focused on increasing teachers' mathematics content knowledge (MCK), with the goal of increasing teacher quality and in turn increasing student mathematics learning. An alternative approach to increasing student mathematics achievement is to investigate curricula that can be effectively used by teachers with a range of MCK. Drawing from a largescale study of kindergarten students (n p 2,598) and their teachers (n p 130), the current study investigated the interaction between teacher MCK and curriculum (Early Learning in Mathematics core kindergarten curriculum vs. business-as-usual curricula) on (a) instructional behaviors and (b) student mathematics achievement gains. Results indicated differential significant interactions across instructional behaviors and a small but negative effect of teacher MCK on student mathematics achievement gains. Implications for future research, policy, and practice are discussed. [This paper was published in "The Elementary School Journal" v123 n2 p292-317 2022.]