Special collections bring together documents that share some common characteristic, such as rarity, format, subject, or provenance. Rare book libraries and archives are two common types of special collections.

Rare book libraries usually collect published sources, but may also collect manuscripts and other unpublished sources, especially literary manuscripts and manuscripts that relate to the library's rare book collections. Rare book libraries are usually organized like other libraries described in this guide, and are often attached to a research or academic library.

Archives, on the other hand, are organized quite differently than libraries. This difference is due to several factors, including the following:

• The massive quantity of documents acquired by archives (an archive will usually measure the size of its collection by linear feet rather than by number of volumes)
• The special nature of archival collections (which sometimes include access restrictions due to privacy rights of people who are the subjects of the documents)
• The evidentiary value of the collection's arrangement as that arrangement was developed by the collection's original creators and users. 

Unlike library collections, which are organized by subject, archival collections are organized by provenance, and to whatever extent possible the archive will attempt "to maintain the integrity of records in relation to their documentary, provenancial, functional, and jurisdictional contexts",¹ by ensuring that the documents remain organized the way they were organized at the time of their creation, or the time of their accession into the archive.

Other types of documents found in special collections include "maps, games, original works of art, realia (nonbook objects, such as furniture, weaponry, or locks of hair), textiles, audiovisual materials, and digital materials".²