This document is the first installment of this Forum (National Forum on Education Statistics) series of guides on longitudinal data systems (LDS). This book focuses on the fundamental questions of what an LDS is (and what it is not), what steps should be taken to achieve a sound system, what components make up an ideal system, and why such a system is of value in education. Chapter 1 introduces this guide series, discussing its purpose, format, and intended audience. Chapter 2 covers some LDS basics, defining the concept of a "longitudinal data system" and laying out key nontechnical steps to planning and developing a successful system. Chapter 3 presents the technical components that generally comprise an LDS, as well as some additional features that may enhance the system. Chapter 4 addresses some common misconceptions regarding longitudinal data systems. Chapter 5 discusses the overarching benefits of an LDS. The appendices include an overview of the four books in this guide series, references, additional resources, a handout summarizing key LDS components, excerpts from LDS-related federal legislation, and other relevant Forum and NCES resources. Appendices include: (1) Overview of the LDS Guide Series; (2) References; (3) Additional Resources; (4) Handout: Anatomy of an LDS; (5) LDS in Legislation; and (6) Forum and Other NCES Resources.