This paper describes "the development and technical adequacy of the Classroom Observations of Student Teacher Interactions (COSTI) instrument," a tool for measuring the frequency and rate of explicit instructional interactions, such as those used in Direct Instruction curricula, for teaching children basic reading and math skills. "The COSTI was originally developed" to provide teachers with coaching feedback to improve their explicit reading instructional practices, "and has been shown in multiple studies to be a reliable and valid predictor of student gains in beginning reading and math skills." This paper discusses potential uses of the instrument for training and coaching across curricula with varying instructional design features, and lays out a future research agenda to further improve the "COSTI" and related observation tools for studying explicit instructional practices and their contribution to student learning. [This paper will be published in "Perspectives on Behavior Science."]