Student achievement depends upon successful passage through a series of transitions: elementary to middle school, middle school to high school, high school to college. Increasing the efficiency of successful transitions requires the collection and analysis of student data across these transition points. Yet long-standing separations between education sectors and continuing proliferation of multiple, disconnected student data systems have served as barriers to statewide tracking of student progress. Recognizing that K-12 and postsecondary data systems must be linked to answer critical questions about student preparation and achievement, many states are currently developing longitudinal data systems; their progress, documented by national studies, is encouraging. However, creating a coherent, effective and sustainable state longitudinal data system requires much more than simply establishing linkages between existing systems. Through a grant from the Bill and Melinda Gates Foundation, SHEEO convened a workshop, "Linking K-12 and Postsecondary Data Systems," as a forum for states to engage in peer-to-peer learning. The workshop brought together cross-sector, data-focused, leadership teams from 11 states with content experts from prominent national organizations for two days of collaborative sessions. Through workshop discussions, state teams identified five core processes which are key to successfully implementing longitudinal data initiatives: (1) Identifying shared benefits as a foundation for cooperative work across sectors; (2) Reconciling technical differences between independently created data systems; (3) Assuring student privacy while sharing data to foster improvement; (4) Designing a data system to enable effective use by key constituencies; and (5) Planning for long-term sustainability of state longitudinal data systems. The development of a statewide longitudinal data system is an enormous endeavor; breaking it down into components can help identify appropriate areas of immediate work. Drawing from the experiences shared by workshop participants--education leaders in states actively engaged in the process of creating seam-less data systems--this report addresses each of these components in turn and presents an overview of insights and strategies to address emerging, prevalent, cross-state concerns. (Contains 7 endnotes.)