The New York Public Library (NYPL) is one of the leading public libraries of the world and is one of the United States's most significant research libraries. It is composed of a very large circulating public library system combined with a very large non-lending research library system. It is simultaneously one of the largest public library systems in the United States and one of the largest research library systems in the world. It is a privately managed, nonprofit corporation with a public mission, operating with both private and public financing. The historian David McCullough has described the New York Public Library as one of the five most important libraries in the United States, the others being the Library of Congress, the Boston Public Library, and the university libraries of Harvard and Yale.