The emergence of longitudinal data systems over the past decade has provided exciting opportunities to better understand students' educational trajectories over time and institutions. The stark reality for today's high school students is that some form of postsecondary education is a necessary ticket of entry to a secure economic future. This guide begins with a brief discussion of the emerging conceptualizations of the precursors of postsecondary success. While many of the constructs in these frameworks are not yet easily measurable, the discussion forms an important backdrop for work designed to increase postsecondary readiness, access, and success. Chapter Two turns to the ways to measure these concepts, based on highly detailed analyses of large datasets on the postsecondary careers of thousands of students. The findings from these studies identify the patterns of interest that can serve to guide school-level analyses of postsecondary data. Chapter Three discusses the primary sources of administrative data that are currently available to schools, along with some of the opportunities and challenges associated with each of these sources. Chapter Four turns to recent efforts to secure detailed data on students' postsecondary outcomes from local institutions. This chapter includes brief profiles of two such efforts, one of which illustrates the tremendous potential of using longitudinal data and the second of which shows what can be done even when longitudinal data for individual students are not available. The next three chapters discuss practical ways to collect and use data. Chapter Five highlights the use of surveys and focus groups to gain additional insight into the findings produced by using longitudinal data. Chapter Six turns to the process of framing questions for inquiry, identifying the data needed to answer them, and making a request for data compliant with legal protection of student records. Chapter Seven discusses the process for working with data at the practitioner level. This chapter concludes with a profile of a consortium of high schools that worked with an outside partner to understand longitudinal data findings on the postsecondary outcomes of their graduates and to learn data-coaching skills that could be used to lead change in each of the schools. [This guide was produced by The Millennium Group International, LLC. This guide was written with Michelle Feist, Patrice Williams, Teri West, and Mary Hastings.]