Under emerging policy structures in California, the responsibility for school improvement is increasingly placed upon local school districts, with County Offices of Education (COEs) playing a critical support role. In this system, districts are responsible for school improvement, with counties in charge of ensuring quality across districts and providing feedback and support where necessary. Underlying this major policy shift is the idea that local leaders are in the best position to drive real educational improvement and ensure quality across multiple schools and contexts. As California supports districts and counties statewide to embark on this improvement journey, there are important lessons to be learned from the CORE districts, six of which developed an innovative accountability system under a waiver from No Child Left Behind (NCLB). The CORE districts are early adopters of the new accountability paradigm: local leaders using multiple measures of school performance and working together to figure out collectively what works best for struggling schools. Now deepening their work together as a Networked Improvement Community (NIC), the CORE districts have simultaneously expanded access to their multiple-measures data and learning system by inviting other California districts to join their "Data Collaborative." Districts who join contribute their own student data to the CORE measurement system and are then able to benchmark student performance against other schools in the state. Data Collaborative districts engage in structured network learning activities that enhance their capacity to use data to drive system improvements. Currently, over 50 California school districts representing nearly a million students have joined the Data Collaborative. This report first provides a framework for how data use for improvement is different from data use for accountability and how data should be used by actors at different levels of the system. Next, it discusses the policy context in California and the current state of data use based on interviews conducted in the summer of 2017 with 41 leaders from state education agencies, COEs, school districts, technical assistance providers, education advocacy organizations, and education associations. Finally, the report shares lessons from the CORE Data Collaborative about how data can be used for improvement within networked structures, including what data is needed and how learning and collaboration can be facilitated. [For "Using Data for Improvement: Learning from the CORE Data Collaborative. Facts-at-a-Glance," see ED594745.]