Explicit instruction is a systematic instructional approach that facilitates frequent and meaningful instructional interactions between teachers and students around critical academic content. This study examined the relationship between student mathematics outcomes and the rate and quality of explicit instructional interactions that take place during core mathematics instruction in kindergarten classrooms using a multifaceted observation system. A total of 379 observations were conducted in 129 classrooms, involving approximately 2,200 students, across a 2-year span. Results suggest that the rate and quality of instructional interactions is related to student mathematics achievement. Implications for instruction and observation research are discussed. [This paper was published in "Elementary School Journal" (EJ1060204).]