The National Longitudinal School Database (NLSD) comprises three files, making up a near-census of all schools and districts in the United States from school years 1990-91 to 2019-20. The three files are the Public School File, Private School File, and District File. As evident by the titles, the first two files report data at the school-level for both public and private schools. The District File reports data at the public school district-level. We have set up these files so that they can be easily merged together in a variety of ways. This data set is unique in that it allows researchers to examine various aspects of school choice across traditional public schools, charter schools, magnet schools, and private schools. These data have been used to examine changes in and effects of charter schools over time (Chen & Harris, 2022) as well as trends and predictors of school closures (Harris & Martinez-Pabon, 2023). This Codebook provides documentation on the sources and methods used to create the first release of the NLSD. It is organized into three sections, corresponding to the three NSLD files (Public, Private, District). Each of the three data files are in a "long" format, such that each row observation provides data for a given school (or school district) in a given school year (1990-1991 through 2019-2020). The accompanying Appendix spreadsheet describes the variables in each data set in more detail, including information on years available, number of observations, ranges, blanks and missing data, and data source. In future releases, we expect to augment the NLSD with additional data beyond the 2019-20 school year and from other sources relevant to its users. Potential developments include integrating data from the Office of Civil Rights, incorporating state-level charter and school take over policies, and exploring methodological refinements to the existing data.