Policymakers and educators need longitudinal data systems capable of providing timely, valid and relevant data. Access to these data gives teachers the information they need to tailor instruction to help each student improve, gives administrators the resources and information to effectively and efficiently manage, and enables policymakers to evaluate which policy initiatives show the best evidence of increasing student achievement. This paper explains the ten essential elements and policy benefits of state longitudinal data. These elements, examined at length herein, are: (1) a unique statewide student identifier; (2) student-level enrollment, demographic and program participation information; (3) the ability to match individual students' test records from year to year to measure academic growth; (4) information on untested students; (5) a teacher identifier system with the ability to match teachers to students; (6) student-level transcript information, including information on courses completed and grades earned; (7) student-level college readiness test scores; (8) student-level graduation and dropout data; (9) the ability to match student records between the pre-K-12 and postsecondary systems; and (10) a state data audit system assessing data quality, validity and reliability. This paper has dealt primarily with actions and issues with global state policy actions related to each of the 10 essential elements. Each of those, however, translates to multiple specific actions that need to occur at the state education agency level. Examples of specific state education agency actions associated with each element are presented in an appendix. [This white paper was produced by the Data Quality Campaign and made possible with the financial support of the Achieve, Inc., American Diploma Project.]