This document, "Book Three of Four: Effectively Managing LDS Data," is the third installment of this Forum series of guides on longitudinal data systems (LDS). This third book in the guide series discusses organizational issues aimed at moving the project forward and ensuring that the data are of high quality so that users may leverage them with confidence for data-driven decisionmaking. The introduction explains the purpose of book three and describes the conventions used throughout the series. Chapter 1 introduces the concept of data governance, and discusses the benefits of this structure and process for data management. Chapter 2 lays out a number of basic steps to establishing effective data governance. Chapter 3 describes the key groups and individual roles that may comprise an education agency's data governance structure. Chapter 4 defines "quality" data. Chapter 5 discusses the various organizational processes that facilitate the creation and maintenance of quality data. Chapter 6 discusses the value of data standards, and describes the major sources of such standards. Chapter 7 provides a basic overview of issues and relevant laws regarding the protection of student data. Chapter 8 addresses the need to secure the LDS data to prevent unauthorized access and tampering. The appendices include an overview of the four books in this series, references, additional resources, a handout on data governance, data governance supplementary materials, and other relevant National Forum on Education Statistics and National Center for Education Statistics (NCES) resources. (Contains 4 figures.) [For "Traveling through Time: The Forum Guide to Longitudinal Data Systems. Book Two of Four: Planning and Developing an LDS. NFES 2011-804," see ED515114.]