Education leaders across the country confront a growing challenge: too many students are not college ready when they leave high school. Although indicators exist to identify students at risk of dropping out of high school, few indicators of students' college readiness are currently in place, and few districts have linked indicators to practices and policies in ways that would enable action to create meaningful, lasting change. The College Readiness Indicator Systems (CRIS) initiative--a collaboration between the Annenberg Institute for School Reform (AISR) at Brown University and the John W. Gardner Center for Youth and their Communities at Stanford University, funded by Bill & Melinda Gates Foundation--aims to address this need for better indicators. Five sites receive support to develop and test college readiness indicators, use them to create effective interventions, and share knowledge and best practices with each other. Data access and integration, in particular, emerged as key issues in early work with the sites. And combining secondary and post-secondary data to trace student outcomes through high school to college is one of the biggest challenges of data integration. One approach to this challenge has been a data-sharing collaboration between the New York City Department of Education (NYCDOE) and the City University of New York (CUNY) to evaluate the college preparedness of their shared students. To support this work, the Bill & Melinda Gates Foundation provided funds to the NYCDOE in 2010 for what is known as the Leaky Pipeline project. In addition to the capacity to analyze data in new ways, the NYCDOE researchers gained many insights into the challenges and best practices of developing and operating a PreK-20 data-sharing collaborative. In this publication, developed in collaboration with AISR as part of the CRIS initiative, these researchers aim to share their insights with others who are engaged in or seek to engage in the work of sh secondary and postsecondary data across institutions to support college readiness. Appended are: (1) Timeline; (2) Additional Metrics on NYCDOE Progress Reports; (3) Effective Data Documentation; and (4) Data-Merging Process. (Contains 2 figures and 2 footnotes.)