The Mastery-Based Learning Collaborative (MBLC) is a demonstration project taking place in 47 schools across Washington to help decision makers understand what quality mastery-based (or competency-based) learning looks like, how long it takes to implement, and what resources are necessary. The schools receive funding, coaching, and professional learning as part of a statewide network. In its first three years, the initiative has led to deeper Mastery-Based Learning (MBL) implementation and has shown positive impacts on early outcomes such as school climate, cultural responsiveness, and student engagement. Almost all school leaders said they would recommend the MBLC to other school leaders. The Aurora Institute is conducting a six-year evaluation of the MBLC to identify policies, practices, and system changes that can support successful mastery-based and culturally responsive learning. The report shares findings from the first three years of the first cohort of MBLC schools, based on extensive surveys and interviews of educators, school leaders, students, state leaders, and professional learning providers. This evaluation report offers insights and strategies for anyone working to shift K-12 schools, districts, and states toward more student-centered, equitable, and culturally responsive approaches.