This document is the fourth and final installment of this Forum series of guides on longitudinal data systems (LDS). One goal of the National Forum on Education Statistics (the Forum) is to improve the quality of education data gathered for use by policymakers and program decisionmakers. An approach to furthering this goal has been to pool the collective experiences of Forum members to produce "best practice" guides in areas of high interest to those who collect, maintain, and use data about elementary and secondary education. Developing LDSs is one of those high-interest areas. These systems hold promise for enhancing both the way education agencies use data to serve students and the way they do business, from the policy level to the school office and into the classroom. This fourth book in the guide series focuses on issues important to the effective use of longitudinal data. The introduction explains the purpose of book four, as well as the format and conventions used throughout the series. Chapter 1 takes a historical look at data use, comparing what was done in the past with what is possible today thanks to detailed longitudinal data. Chapter 2 describes several preconditions to effective data use. Chapter 3 summarizes the many uses of longitudinal data, and the types of users who can leverage the information. Chapter 4 offers tips on implementing effective training and professional development programs on the use of longitudinal data. Chapter 5 discusses the road ahead for LDS development and use. Appended are: (1) Overview of the LDS Guide Series; (2) References; (3) Additional Resources; and (4) Forum and Other NCES Resources. (Contains 1 table and 2 figures.) [For related reports, see the following: (1) "Traveling through Time: The Forum Guide to Longitudinal Data Systems. Book One of Four: What Is an LDS? NFES 2010-805" (ED511638); (2) "Traveling through Time: The Forum Guide to Longitudinal Data Systems. Book Two of Four: Planning and Developing an LDS. NFES 2011-804" (ED515114); and (3) "Traveling through Time: The Forum Guide to Longitudinal Data Systems. Book Three of Four: Effectively Managing LDS Data. NFES 2011-805" (ED515850).]