Formative assessment has been shown to be a critical activity for promoting meaningful understanding in the classroom. Systematic engagement in formative assessment helps instructors to develop a clearer picture of where students stand in relation to the learning objectives of the course. However, college instructors teaching large-enrollment STEM (science, technology, engineering, and mathematics) courses face diverse challenges when trying to engage in formative assessment of student understanding. This article describes how the participation in the classroom of an undergraduate student acting as a learning researcher (LR) can help instructors focus their attention on formative assessment and productively use it to support and advance student learning. LRs in our project provide real-time formative feedback to instructors through daily reports based on classroom observations. They enable systematic formative assessment by providing quality descriptions and interpretations of student thinking, and productive suggestions for improving instruction. The work of LRs has positively affected instructional practice in a variety of courses across diverse STEM disciplines in our institution.