The reference desk or information desk of a library is a public service counter where professional librarians provide library users with direction to library materials, advice on library collections and services, and expertise on multiple kinds of information from multiple sources.

Library users can consult the staff at the reference desk for help in finding information. Using a structured reference interview, the librarian works with the library user to clarify their needs and determine what information sources will fill them.

The ultimate help provided may consist of reading material in the form of a book or journal article, instruction in the use of specific searchable information resources such as the library's online catalog, or subscription bibliographic/full text databases, or simply factual information drawn from the library's print or online reference collection. Information is also provided to patrons through electronic resources. Typically, a reference desk can be consulted either in person, by telephone, through email or online chat, although a library user may be asked to come to the library in person for help with more involved research questions. A staffed and knowledgeable reference desk is an essential part of a library.

The services that are provided at a reference desk may vary depending on the type of library, its purpose, its resources, and its staff. The services of circulation and reference desks are also sometimes combined.

Reference services did not become commonplace in libraries until the late 1800s. These services initially began in public libraries. At first librarians were hesitant to offer reference services because many libraries did not have a large enough staff to provide the services without other duties being neglected. Beginning in 1883 with the Boston Public Library, libraries began to hire librarians whose primary duty was to provide reference services.

One of the earliest proponents of references services was Samuel Swett Green. He wrote an article titled "Personal Relations Between Librarians and Readers" which had a large impact on the future of reference services. Utor (2008) defined reference services as a direct personal assistance to readers seeking information through direct contact between the reference librarian and the user.

Reference desks changed dramatically with the emergence of information technology. During the 2020 pandemic, there was a surge of virtual information desks. It has been found that chat, email, and web conferencing formats have become adequate substitutions for in-person conversations. AI chatbots are also being used in academic libraries to address typical information inquiries, although their use is still being tested.

Resources that are often kept at a library reference desk may include: