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The group of objects owned by a museum

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.

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.

Once accessioned into the collection, museum objects must be appropriately cared for. New objects may be examined by a conservator and treated for any pre existing damage. The object is then cataloged by a curator or other specialist with knowledge of the object's importance and history. The object will then be given an appropriate storage location. Museum storage conditions are meant to protect the object and to minimize any deterioration. This often means keeping objects in a stable climate, preventing exposure to pests, minimizing any handling, and using only archival materials that will not deteriorate or harm the objects. Object safety also include providing appropriate security, and planning for disasters and other threats, and making sure that museum staff are trained in proper handling procedures. Different types of objects have different requirements, and many museums have specialized storage areas. For example, framed paintings may be stored in racks in one room while unframed paintings are kept in large drawers in another. Some objects have extremely specialized needs. For example, material from underwater archaeological sites may need to be kept wet, and some very rare and badly deteriorated objects require oxygen free environments. At any given time, museums display only a portion of their collections. This is often because exhibition requires much more space than storage, and is impractical for the entire collection to be out. Museums may also contain many duplicate or similar objects and find that a few specimens are better suited to display than others. In addition, certain objects, particularly works on paper and textiles, are damaged by light and must only be displayed for short periods of time. Museum collections are often made up of a variety of materials in a single collection including, but not limited to: canvas, oil and/or acrylic paints, wood, ivory, paper, bone, leather, and textiles. The biggest conservation issue for museum collections is the fluctuations in relative humidity and temperature. Relative Humidity (RH) is a measure of the percentage of saturation of the air. Temperature is not as important to the life of a work of art, but it is true that chemical reactions occur faster at higher temperatures. However, a museum must take into account the comfort of its staff and visitors and it has been widely accepted that does not cause a lot of problems for most artifacts and is comfortable for most humans. This has become widely accepted because the lower limit was set at 45% since damage to organic materials begin to occur below this point. The upper limit is placed at 65% because mold flourishes at 70% RH. It is also cheaper for most institutions to maintain 50% RH rather than 45% or 60%. There is some exception when it comes to tropical climates since the indigenous artifacts are acclimated to RH levels higher than the "museum norm". Another case was the sale of a J. M. W. Turner painting in the collection of Royal Holloway, University of London to the Getty Museum to fund the maintenance of the building, despite the fact that the original benefactor had expressly requested that the collection be kept intact. Many ethical guidelines for deaccessioning require that the funds generated by disposing of collection items be used only to increase or maintain the remaining collection. For example, the International Council of Museums (ICOM) Code of Ethics states that:

"Money or compensation received from the deaccessioning and disposal of objects and specimens from a museum collection should be used solely for the benefit of the collection and usually for acquisitions to that same collection". In the United Kingdom, guidelines governing deaccessioning and other ethically difficult issues can be found in the Museums Association's Code of Ethics. In the United States, the guidelines on these matters are issued by the American Alliance of Museums. The American Alliance of Museums Code of Ethics takes the position that "in no event shall they [deaccessioning proceeds] be used for anything other than acquisition or direct care of collections".