Music catalogers have long considered the Music Cataloging Bulletin an indispensable tool. With this 10-year cumulative index and supplement, catalogers have a convenient, extensive subject index and a practical way to review changes to the classification schedules and subject headings made during the 1990s. This publication comprises the fifth cumulative index and supplement to the Music Cataloging Bulletin (MCB), a monthly publication of the Music Library Association. It covers the information contained in volumes 21-30 (1990-1999) and is in five sections, mirroring the organization of information in the Bulletin. The subject index section expands on the annual indexes to include entries for all names, committees, task forces, and publications, as well as cataloging and MARC tagging changes mentioned in the monthly issues, including the two years where no annual index was issued (1994 and 1999). The index identifies acronyms and abbreviations and places entries in their organizational hierarchy when appropriate. In the section containing the Library of Congress Classification additions and changes, each entry appears in its complete hierarchical context. A separate section covers additions, changes and cancellations to music-related Library of Congress Subject Headings, as reported in these issues of MCB. This section includes cross-references from a cancelled heading to its replacement. Additional sections include a list of all new reference work titles added to the Library of Congress'' Music Section during this time period, and a compilation of changes or additions to thematic indexes used in formulating uniform titles for music, as reported by the Library of Congress.