An academic library is a library that is attached to a higher education institution and serves two complementary purposes: to support the curriculum and the research of the university faculty and students. It is unknown how many academic libraries there are worldwide. An academic and research portal maintained by UNESCO links to 3,785 libraries. According to the National Center for Education Statistics, there are an estimated 3,700 academic libraries in the United States. In the past, the material for class readings, intended to supplement lectures as prescribed by the instructor, has been called reserves. In the period before electronic resources became available, the reserves were supplied as actual books or as photocopies of appropriate journal articles. Modern academic libraries generally also provide access to electronic resources.
Academic libraries must determine a focus for collection development since comprehensive collections are not feasible. Librarians do this by identifying the needs of the faculty and student body and the mission and academic programs of the college or university. When there are particular areas of specialization in academic libraries, these are often referred to as niche collections. These collections are often the basis of a special collection department and may include original papers, artwork, and artifacts written or created by a single author or about a specific subject.
There is a great deal of variation among academic libraries based on their size, resources, collections, and services. The Harvard University Library is considered to be the largest strict academic library in the world, although the Danish Royal Library—a combined national and academic library—has a larger collection. Another notable example is the University of the South Pacific which has academic libraries distributed throughout its twelve member countries. The University of California operates the largest academic library system in the world, managing more than 34 million items in 100 libraries on ten campuses.
The first colleges in the United States were intended to train members of the clergy. The libraries associated with these institutions largely consisted of donated books on the subjects of theology and the classics. In 1766, Yale had approximately 4,000 volumes, second only to Harvard. Access to these libraries was restricted to faculty members and a few students: the only staff was a part-time faculty member or the president of the college. The priority of the library was to protect the books, not to allow patrons to use them. In 1849, Yale was open 30 hours a week, the University of Virginia was open nine hours a week, Columbia University four, and Bowdoin College only three. Students instead created literary societies and assessed entrance fees in order to build a small collection of usable volumes, often in excess of what the university library held.
Around the turn of the century, this approach began to change. The American Library Association (ALA) was formed in 1876, with members including Melvil Dewey and Charles Ammi Cutter. Libraries re-prioritized in favor of improving access to materials and found funding increasing as a result of increased demand for said materials.
Academic libraries today vary in regard to the extent to which they accommodate those who are not affiliated with their parent universities. Some offer reading and borrowing privileges to members of the public on payment of an annual fee; such fees can vary greatly. The privileges so obtained usually do not extend to such services as computer usage, other than to search the catalog, or Internet access. Alumni and students of cooperating local universities may be given discounts or other considerations when arranging for borrowing privileges. On the other hand, access to the libraries of some universities is absolutely restricted to students, faculty, and staff. Even in this case, they may make it possible for others to borrow materials through inter-library loan programs.
Libraries of land-grant universities generally are more accessible to the public. In some cases, they are official government document repositories and so are required to be open to the public. Still, members of the public are generally charged fees for borrowing privileges, and usually are not allowed to access everything they would be able to as students.
Academic libraries in Canada are a relatively recent development in relation to other countries. The very first academic library in Canada was opened in 1789 in Windsor, Nova Scotia. Academic libraries were significantly small during the 19th century and up until the 1950s when Canadian academic libraries began to grow steadily as a result of greater importance being placed on education and research. The growth of libraries throughout the 1960s was a direct result of many overwhelming factors including, inflated student enrollments, increased graduate programs, higher budget allowance, and general advocacy of the importance of these libraries. As a result of this growth and the Ontario New Universities Library Project that occurred during the early 1960s, five new universities were established in Ontario that all included fully catalogued collections. The establishment of libraries was widespread throughout Canada and was furthered by grants provided by the Canada Council and the Social Sciences and Humanities Research Council, which sought to enhance library collections. Since many academic libraries were constructed after World War Two, a majority of the Canadian academic libraries that were built before 1940 that have not been updated to modern lighting, air conditioning, etc., are either no longer in use or are on the verge of decline. The total number of college and university libraries increased from 31 in 1959–1960 to 105 in 1969–1970.
Following the growth of academic libraries in Canada during the 1960s, there was a brief period of sedation, which was a primary result of some major budgetary issues. These academic libraries were faced with cost issues relating to the recently developed service of interlibrary lending and the high costs of periodicals on acquisition budgets, which affected overall acquisition budgeting and, ultimately general collections. Canadian academic libraries faced consistent problems relating to insufficient collections and an overall lack of coordination among collections.
Academic libraries within Canada might not have flourished or continued to be strengthened without the help of outside organizations. The Ontario Council of University Libraries (OCUL) was established in 1967 to promote unity among Canadian academic libraries. The Ontario College and University Library Association (OCULA) is attached to the Ontario Library Association (OLA) and is concerned with representing academic librarians regarding issues shared in the academic library setting.
Academic libraries have transformed in the 21st century to focus less on physical collection development and more on information access and digital resources. Today's academic libraries typically provide access to subscription-based online resources, including research databases and ebook collections, in addition to physical books and journals. Academic libraries also offer space for students to work and study, in groups or individually, on "silent floors" and reference and research help services, sometimes including virtual reference services.  Some academic libraries lend out technology such as video cameras, iPads, and calculators. To reflect this changing focus, many academic libraries have remodeled as Learning Commons. Academic libraries and learning commons often house tutoring and writing centers and other academic services.
A major focus of modern academic libraries is information literacy instruction, with most American academic libraries employing a person or department of people dedicated primarily to instruction. Many academic institutions offer faculty status to librarians, and librarians are often expected to publish research in their field. Academic librarian positions in the United States usually require an MLIS degree from an ALA-accredited institution. The Association of College and Research Libraries is the largest academic library organization in the United States.