This article explores the development and evaluation of the Building Leadership Team Assessment Tool (BLT-AT), designed to measure Professional Learning Communities' (PLCs') use of effective school improvement practices. The BLT-AT is grounded in Ohio's inclusive instructional leadership model, which emphasizes the improvement of teaching and learning, capacity building through support and accountability, an open and collaborative culture, and systemwide learning. The work to develop the BLT-AT aimed to provide schools with a reliable, open-access tool for assessing and monitoring their school improvement practices. A practice profile, which can help PLCs debrief BLT-AT results, also accompanies the instrument. Data for testing validity claims about the instrument came from a survey of PLC (and potential PLC) members across various Ohio schools, predominantly located in suburban areas. The study used exploratory factor analysis, which resulted in the identification of three key constructs: culture of continuous improvement, culture of shared leadership, and culture of inclusiveness. Scales constructed to reflect each of the factors also showed high internal-consistency reliability. Overall, the findings suggest that the BLT-AT is a promising instrument for measuring school-level improvement practices, as described in contemporary literature on educational leadership and reform.