Recent instructional reforms in science education bring a new vision that aims to move science teaching away from a focus on discrete facts covered at a superficial level to a focus on a smaller number of disciplinary core ideas that can be explored in depth. The success of these reforms depends on the instructional quality in science classrooms. Our goal in this paper is to introduce an instructional quality assessment tool that can provide a window into the extent to which students learn science through rigorous opportunities as envisioned in these reforms. We present the conceptual grounding of the tool and its development process with findings from a pilot test. Data collection involved in-class assignments, students' work, and video-records from nine science teachers' classrooms at a local school. The analysis revealed that the instructional quality assessment tool can reliably capture the variation in rigor in science classrooms. Specifically, the tool has the potential to assess elements of ambitious science teaching focusing on the rigor of tasks and talk that shape students' high-level thinking and sensemaking in science classrooms. The article concludes by discussing future directions for the further development of this tool and the ways in which it can be used for instructional improvement and teacher learning.