The pedagogy of mastery learning requires students to demonstrate a deep understanding of a given course topic before being allowed to proceed to the next topic. Teachers use formal or informal assessments to monitor students' progress, providing additional support to students whose performance shows they have not mastered the content. Students retake assessments as necessary until they do demonstrate mastery. There is no penalty for needing extra attempts. Mastery learning has been around for several decades, and much has been written about its effectiveness, reporting varying results. Much less has been written about how schools implement mastery learning. This report describes how three Kentucky high schools in different districts, each working independently with the University of Kentucky to reform their educational practices, define, implement, and report on their mastery learning programs. The schools represent a variety of educational settings and range of experience with mastery learning. Through interviews and document analysis, this report examines four broad questions about mastery learning in the participating high schools: (1) How do the schools define and measure mastery learning? (2) Why did the schools choose mastery learning?; (3) How do the schools implement mastery learning, including use of assessments, instructional differentiation, role of technology, and professional development?; and (4) What beliefs do stakeholders have about the successes and challenges associated with the schools' mastery learning implementation?