A museum is distinguished by a collection of often unique objects that forms the core of its activities for exhibitions, education, research, etc. This differentiates it from an archive or library, where the contents may be more paper-based, replaceable and less exhibition oriented, or a private collection of art formed by an individual, family or institution that may grant no public access. A museum normally has a collecting policy for new acquisitions, so only objects in certain categories and of a certain quality are accepted into the collection. The process by which an object is formally included in the collection is called accessioning and each object is given a unique accession number.
Museum collections, and archives in general, are normally catalogued in a collection catalogue, traditionally in a card index, but nowadays in a computerized database. Transferring collection catalogues onto computer-based media is a major undertaking for most museums. All new acquisitions are normally catalogued on a computer in modern museums, but there is typically a backlog of old catalogue entries to be computerized as time and funding allows.
A museum's permanent collection are assets that the museum owns and may display, although space and conservation requirements often mean that most of a collection is not on display. Museums often also host temporary exhibitions of works that may come all or partly from their permanent collection, or may be all or partly loaned (a "loan exhibition"). A travelling exhibition is shown in more than one venue; these tend to be either large loan exhibitions which may be exhibited at two or three venues in different countries, or selections from the collection of a large museum which tour to a number of regional museums.
Types
Museum collections are widely varied. There are collections of art, of scientific specimens, of historic objects, of living zoological specimens and much more. Because there are so many things to collect, most museums have a specific area of specialization. For example, a history museum may only collect objects relevant to a particular county or even a single person, or focus on a type of object such as automobiles or stamps. Art museums may focus on a period, such as modern art, or a region. Very large museums will often have many sub-collections, each with its own criteria for collecting. A natural history museum, for example, will have mammals in a separate collection from insects.
Because museums cannot collect everything, each potential new addition must be carefully considered as to its appropriateness for a given museum's defined area of interest.
Accessioning
Accessioning is the formal, legal process of accepting an object into a museum collection. Because accessioning an object carries an obligation to care for that object in perpetuity, it is a serious decision. While in the past many museums accepted objects with little deliberation, today most museums have accepted the need for formal accessioning procedures and practices. These are typically set out as part of a museum's collection management policy (CMP).
While each museum has its own procedures for accessioning, in most cases it begins with either an offer from a donor to give an object to a museum, or a recommendation from a curator to acquire an object through purchase or trade. Art objects may also come into a collection as a commission. An accession may also be bequeathed to a museum and are included in an estate or trust.
Several issues must be considered in the decision to accept an object. Common issues include:
Is the object relevant to the museum's mission and its scope of collecting, as defined by its governing body? Was the object lawfully acquired and if foreign in origin, imported in compliance with international law? Does the owner of an object have legal title to the object and therefore the right to transfer it? Are there any other parties with an interest in the object (e.g. heirs of a donor, descendant groups for cultural objects, etc.)? Is the object encumbered by any legal obligations or constraints (e.g., natural history objects that require special permits)? Would the object pose any threats or dangers to other objects or staff? Does the museum have the resources to properly care for the object (e.g., appropriate storage space, adequate funding) Is the object encumbered by any donor restrictions?
Answering these questions often required investigating an object's provenance, the history of an object from the time it was made.