Isadore Gilbert Mudge (March 14, 1875 – May 16, 1957) was ranked by the magazine American Libraries as one of the top 100 important leaders that libraries have had in the 20th Century. Mudge was a defining influence on what a contemporary reference librarian is and was essential for helping organize and promote reference books for all libraries. She is known both as the foremost reference librarian of her time and an author and editor of various reference books.