This article outlines the development of an assessment tool for charting student learning through inquiry and provides some insights into its use as an evidence-based practice tool for school library instructional teams. The tool, known as the Student Learning through Inquiry Measure (SLIM), has been developed and tested over a number of years by researchers at the Center for International Scholarship in School Libraries (CISSL). The SLIM toolkit enables instructional teams to chart the changes in knowledge of students of their curriculum topics as they progress through the stages of their inquiry task. It gives insights into the substance of their knowledge, the amount of knowledge, the structure of their knowledge, and how students perceive their knowledge changes as they progress through the stages of a guided inquiry project.