Principals should support school libraries because it is in both their students' and their own best interest to do so. Quality library media programs can enhance student achievement, and informed, committed librarians can help principals enhance their own administrative practice. This ERIC Digest discusses improving student achievement, including research identifying positive correlations between student achievement and library media services and 11 media services program characteristics; administrator support; and how principals can support libraries. The Digest concludes by stating that principals interested in developing their libraries as instruments of school improvement can ask their librarians to assemble a research collection to share with board members, district administration, and faculty. (Contains 27 references.) (AEF)