There is no shortage of data in the education system today. States and school districts are gathering large amounts of information on school and student performance. The challenge now is to make sure states are collecting the most relevant data and that they have a coordinated system for using them effectively. To provide educators with the data they need to improve student achievement, states need more than a series of one-time snapshots of student performance. They need a system that collects high-quality data about how individual students are doing over time, from prekindergarten through 12th grade and into postsecondary education. This information -- also known as longitudinal data -- makes it possible to: (1) follow students' academic progress as they move from grade to grade; (2) determine the value-added and efficiencies of specific schools and programs; (3) identify consistently high-performing schools so that educators and the public can learn from best practices; (4) evaluate the effect of teacher preparation and training programs on student achievement; and (5) focus school systems on preparing a higher percentage of students to succeed in rigorous high school courses, college and challenging jobs. To take full advantage of the power of longitudinal data, states need systems that can exchange information within and across pre-K-12 and postsecondary systems and states. This will allow states to continue monitoring achievement as students move from place to place and through the education pipeline. Ten essential elements of a longitudinal data system are detailed and future directions of state data systems are discussed.