The National and University Library (Narodna in univerzitetna knjižnica), whose archives contain, among other things, a rich collection of medieval manuscripts, incunabula and Renaissance prints, was built between 1936 and 1941. The building, which represents Slovenia's most important work by architect Plecnik, is located on the site of the former Ducal Palace, a 17th century Baroque palace destroyed in the earthquake of 1895.

The exterior of the building reflects Italian influences. The characteristic façade combining bricks and stone blocks in variable finishes was designed on the model of the house of the Roman artist Federico Zuccari. The entrance door handle is decorated with two horse heads representing Pegasus, a winged horse, which is supposed to symbolize the visitors' guide into the realm of knowledge. From the entrance hall, a door leads to the famous monumental central staircase with 32 pillars of the black Podpec marble, and the staircase to the library's grand reading room. The particularly interesting library details include Plecnik's chandeliers and two glass walls, which allow light to reach the wooden reader desks and books from two sides.

Above the side entrance to the library, there is a bronze statue of Moses by the Slovenian sculptor Lojze Dolinar. To indicate the course of the Roman and medieval town walls, which used to run alongside the library building, Plecnik erected a line of monuments to famous Slavicists in front of the library, and a line of monuments to Slovenian composers in front of the Philharmonic Society building (Glasbena Matica), which is sited across the street from the library.