The National and University Library holds approximately 2,8 million items representing three basic categories: the main national collection, a collection of important scientific literature, and a collection of special library materials. The national collection, or Slovenika, is the core of the Slovenian written heritage. It includes all works which were either edited or published in Slovenia, refer to Slovenia, and were written in the Slovenian language, or written by Slovenian authors. The Slovenika collection was given the status of a cultural monument and its preservation is the key task of the National Library. Owing to the principle of legal deposit, the National Library receives copies of all publications produced in Slovenia. Although the Slovenika has been part of the Library acquisition policy for more than 200 years, also due to conscious librarians, the Library acquired this function only in 1945.
The same applies to the collection of fundamental scientific literature which originates from the end of the 18th century. The Lyceum Library, the forerunner of today's National and University Library, was designed and established as the first public scientific library in the Slovenian territory. Its main aim was to serve the academic and research needs of professors and students of the newly founded Lyceum, as well as other scholars. The main library policy until the end of the Second World War was – both formally and in practice - to upgrade the library collections by acquiring relevant scientific and academic literature for the study programmes of the Ljubljana high, higher and secondary schools on the one hand, and to serve wider needs of intellectuals on the other.
Special library collections are also intended for in-depth scientific research and academic work. Along with their regular tasks, national libraries usually create special collections where librarians have an excellent overview of specific sections. There are six special collections in NUK: the Manuscripts, Rare and early Publications Collection, the Music Collection, the Map and Pictorial Collection, the Ephemera and Grey Literature Collection, the Serials Collection and the Slovenian Diaspora Publications Collection.