The Library of Congress is the premiere institution for research on the life and works of Leonard Bernstein. This chapter details how Bernstein’s relationship with the library was established, documents the donations he made to the library during his lifetime, and describes how the relationship with the Bernstein estate has continued to thrive. This chapter gives an overview of the richness of the library’s Leonard Bernstein Collection, one of the most exceptional in the Music Division in terms of the variety and scope of material it contains. In addition to documenting Bernstein’s work and creative process, the Bernstein Collection provides countless avenues of research for those studying music history, television history, education, the Civil Rights movement, LGBTQ+ topics, Jewish identity, and pop culture. The chapter also highlights relevant archival material found elsewhere in the Music Division’s collections, and connects readers with digitized collection material available on the Library of Congress website.