The library is open to the general public for academic research, and runs tours for visitors. Only those who are issued a "Reader Identification Card" may enter the reading rooms and access the collection. The Reader Identification Card is available in the Madison building to persons who are at least 16 years of age upon presentation of a government issued picture identification (e.g., driver's license, state ID card or passport). However, only members of Congress, Supreme Court Justices, their staff, Library of Congress staff and certain other government officials can actually remove items from the library buildings.

Members of the general public with Reader Identification Cards must use items from the library collection inside the reading rooms only; they cannot remove library items from the reading rooms or the library buildings. During the 110th Congress, in the Fall, United States House of Representatives Pages were given Library of Congress Reader Cards for the first time in the Page Program's history, giving them access to the large reading rooms available in the Library of Congress. Pages can be as young as 16 years of age.

Since 1902, libraries in the United States have been able to request books and other items through interlibrary loan from the Library of Congress if these items are not readily available elsewhere. Through this, the Library of Congress has served as a "library of last resort", according to former Librarian of Congress Herbert Putnam.