In this article, we describe a framework and instrument for measuring the mathematical quality of mathematics instruction. In describing this framework, we argue for the separation of the "mathematical quality of instruction" (MQI), such as the absence of mathematical errors and the presence of sound mathematical reasoning, from pedagogical method. We argue that conceptualizing this key aspect of mathematics classrooms will enable more clarity in mathematics educators' research questions and will facilitate study of the mechanisms by which teacher knowledge shapes instruction and subsequent student learning. The instrument we have developed offers an important first step in demonstrating the viability of the construct.