Individual theft

Many thieves are motivated by the fact that reasonably valuable art pieces are worth millions of dollars and weigh only a few kilograms, at most. Transportation is also trivial, assuming the thief is willing to inflict some damage to the painting by cutting it off the frame and rolling it up into a tube carrier. While most high-profile museums have extremely tight security, many places hosting multimillion dollar works have disproportionately poor security measures.That makes them susceptible to thefts that are slightly more complicated than a typical smash-and-grab, but with huge payoff. However, because the ownership of high profile art is easily tracked, potential buyers are very hard to find. Typically, a thief will steal a work, only to find out that there are no buyers. For the same reason, the stolen piece cannot be put on display publicly, which essentially defeats the purpose of having it. Unfortunately, while no thief can hope to get the actual value of the stolen work, even as little as 5% of the real value can be worthwhile for the thief. Most art is resold at auction houses; major reputable houses such as Sotheby's or Christie's demand proof of art ownership before listing. Many lost art pieces that become found and sold at auction have later been exposed as forgery or imitation.
A likely scenario in famous art theft is "theft for hire" or similar situations in which buyers have already been found. Some buyers may enjoy possessing famous art secretly. Fossil theft is an easier form of purchase as identification techniques are not as well established as art theft.

State theft, wartime looting and misappropriation by museums
Because antiquities are often regarded by the country of origin as national treasures, there are numerous cases where artworks (often displayed in the acquiring country for decades) have become the subject of highly charged and political controversy. One prominent example is the case of the Elgin Marbles, which were removed from Greece to the British Museum in 1816 by Thomas Bruce, 7th Earl of Elgin. Many different Greek governments have maintained that removal was tantamount to theft
Similar controversies have arisen over Etruscan, Aztec and Italian artworks, with advocates of the originating countries generally alleging that the removal of artifacts is a pernicious form of cultural imperialism. Yale University's Peabody Museum of Natural History is engaged (as of November 2006) in talks with the government of Peru about possible repatriation of artifacts taken during the excavation of Machu Picchu by Yale's Hiram Bingham.
In 2006, New York's Metropolitan Museum reached an agreement with Italy to return many disputed pieces. The Getty Museum in Los Angeles is also involved in a series of cases of this nature. The artwork in question is of Greek and ancient Italian origin. The museum agreed on 20 November 2006 to return 26 contested pieces to Italy. One of the Getty's signature pieces, a statue of the goddess Aphrodite, is the subject of particular scrutiny.
From 1933 through the end of World War II, the Nazi regime maintained a policy of looting art for sale or for removal to museums in the Third Reich. Hermann Goering, head of the Luftwaffe, personally took charge of hundreds of valuable pieces, generally stolen from Jews and other victims of genocide. Members of the families of the original owners of these artworks have, in many cases, persisted in claiming title to their pre-war property. In 2006, title to the gilt painting Portrait of Adele Bloch-Bauer I, by Austrian artist Gustav Klimt, was restored to Maria Altmann, an heir of the prewar owner. Provenance in this case was easy to establish; Bloch Bauer, the subject of the painting, was Altmann's aunt. Altmann almost immediately sold the painting at auction and it was resold to Ronald Lauder for $135 million. At the time of the latter sale this was the highest known price ever paid for a painting.