ABSTRACT The passage of Act 77 in June 2013 and the Educational Quality Standards passed in April 2014 are significantly changing the way education is being conceptualized in Vermont. These two policy mandates called for all Vermont high schools to shift to proficiency-based learning (PBL), also known as standards-based, competency-based or mastery-based learning, by 2020. Yet scant research exists on how to implement PBL. This qualitative study addresses this need by examining the perspectives of three exemplary high school social studies teachers who were early adopters of proficiency- based instruction and learning in their classrooms. The research centered on questions about the teachers' perspectives on the curricular, instructional, and assessment shifts accompanying and supporting the implementation of PBL. The theoretical framework that informed this study was constructivist theory and the notion that knowledge is socially constructed through the learner's interaction with the world (Brooks & Brooks, 1999). The study focused on teachers' perspectives on and understandings of these shifts in order to capture innovative tools, strategies, and instructional approaches they developed as they implemented PBL. The findings may inform the thinking of social studies educators, administrators, policy makers, students, and other stakeholders interested in implementing PBL. The major findings that emerged in this study included several key components the teachers identified as vital to PBL implementation in a classroom including the need to: 1) identify key skills and concepts required to meet proficiencies, 2) use targeted and ongoing feedback with learners, 3) enact a curricular design that situates proficiencies in authentic experiences that provide multiple opportunities for practice, 4) support the emergence of new structures in high schools such as larger chunks of time with students, high school teaming, and flexible grouping of students, and 5) teach students explicitly about the learning process. The study also identified several broader policy considerations related to the implementation of PBL including a need for: 1) targeted professional development, 2) restructured school schedules to accommodate collaborative learning conversations among educators, administrators, and students, 3) collaboratively designed (including student voice) learning proficiencies that create a coherent experience from grades 9 to 12 (Fullan, 2016), and 4) redesigned preservice teacher training so that newly credentialed teachers are prepared to teach in proficiency-based centered learning environments.