In 2010, California adopted the Common Core State Standards (CCSS) for math and English. The new standards are part of a state effort to prepare students for college and careers in the 21st-century global economy and narrow longstanding achievement gaps. The state's implementation is complete, but because districts decide whether and when they will adopt the standards, far less is known about local efforts. In early 2019, the authors surveyed school districts to gauge their progress on Common Core implementation. They then used statewide data and examined the differences in local textbook adoption to understand the effects of CCSS implementation on student outcomes. This report details the findings: (1) Progress is uneven; (2) Most districts implemented recently; (3) Instructional practices lag behind; (4) There are modest improvements among elementary and middle school students in districts that have adopted the standards; and (5) The impact of the standards in high schools is inconclusive. These findings point to several steps the states can take to help districts implement the standards: (1) Invest in programs to monitor local implementation and provide continuous support; (2) Provide more guidance about quality, content-based, professional development, especially for math teachers; and (3) Collaborate with researchers and educators to identify effective system shifts that may be scalable across the state. [This report was written with research support from Courtney Lee. For the technical appendices to this report, "Common Core State Standards in California: Evaluating Local Implementation and Student Outcomes. Technical Appendices," see ED603728.]