State education data systems help policymakers use data to evaluate the impact of their efforts to improve education. By disaggregating the data -- that is, breaking it out by different student subgroups -- policymakers can ensure that their efforts address the needs of students who have been traditionally underserved in educational settings. Yet barriers to collecting and reporting on disaggregated data persist. Nonetheless, states that disaggregate data as fully as possible can best support the increasingly diverse student populations they serve in their schools and higher education institutions. Past and present data collection practices are presented in this Policy Brief along with insight on enduring challenges and strategies for improving both the collection and presentation of data. The brief is the second installment in a three-part series on how the use of an equity framework can support success for students who are underserved in educational settings. [For "Creating an Equity Framework That Supports Underserved Students. Policy Brief," see ED620218. For "Promising Practices in Creating an Equity Framework. Policy Brief," see ED620215.]