The focus of this paper--and the series of which it is a part--is on improving student learning during the crucial period spanning grade 12 and the first year of college in order to build momentum for college success. In the introductory paper to this series, "Why 12th Grade Must Be Redesigned Now--and How," Jobs for the Future names this period the "transition zone" and argues that it is time for high schools and colleges to consider taking joint responsibility for the college and career readiness of students during these two years. This series suggests a "shared transition zone," in which secondary and postsecondary education systems and institutions would collaborate in key ways to bridge existing gaps and substantially increase the percentage of youth prepared for college and careers. While high schools and colleges have their own distinct roles in educating students--and are trying to make improvements in their respective systems--their shared interest in student success comes closest to converging at the end of high school and the beginning of college. The needs of young people leaving one institution and entering another are quite similar, but the leaps in academic and cultural expectations are quite large. We suggest that it is at this juncture where sharing responsibility for the same students in a shared transition zone can have a greater effect than acting apart. In this paper, the authors outline the principles of co-design, co-delivery, and co-validation that must guide the new partnerships between high school and college campuses and systems to raise college readiness and success. They describe the practices of exemplary partnerships around the country and suggest policies to promote the development of more partnerships that can spread this innovative work. [For the first paper in this series, "Why 12th Grade Must Be Redesigned Now--and How," see ED560768.]