The shift to the Common Core at the Lazaro Cardenas Elementary School, a pre-K to third grade school in a predominantly Latino area in southwest Chicago, rested on instructional improvement efforts Jeremy Feiwell initiated five years earlier in 2006, when he became Cardenas's principal. Feiwell knew from his years as a teacher at Cardenas that curriculum and instruction at the school varied widely--every teacher was doing something different in his or her classroom. Meanwhile, the school--where 52 percent of students are English language learners and 97 percent come from low-income households--was the lowest performing of the 23 schools in the community. By 2016, the school was the highest performing in math in the south side of Chicago, and one of the top schools for reading. Cardenas's staff attributes a significant portion of the school's success in recent years to their work developing a curriculum that meets the demands of higher standards. Over the course of his tenure, Feiwell has established high expectations for all students, worked with staff in adopting a coherent school-wide curriculum, and intentionally built teacher capacity and responsibility for continually improving the content and delivery of this curriculum. In this case study, the authors describe the three phases of curricular improvement undertaken by Feiwell and his staff. The study concludes with questions for reflection. [For the main report, "Ambitious Leadership: How Principals Lead Schools to College and Career Readiness," see ED584804.]