What makes an artwork real? This seemingly straightforward question can actually be quite loaded. A faked artwork is generally considered to be a copy of an existing work, but what about when an artist copies their own? A forgery, on the other hand, imitates an artist's style to create a new work. But when the forged artist was challenging artistic originality, the line gets blurry.

Museums are not impervious to these phenomena. The Metropolitan Museum of Art, for example, has a 1624 portrait of Spain's King Philip IV that it deemed in the 1970s to have been created by an assistant to Diego Velázquez, only to reauthenticate it fully to the Spanish Baroque painter in 2010. Experts lean on the three-legged stool of art authentication: stylistic scrutiny, provenance, and scientific analysis.

Below are nine of the most faked and forged artists. Jean-Baptiste-Camille Corot, for instance, saw his work copied extensively between 1870 and 1939. A single Belgian studio manufactured 235 fake Corot landscapes in 1888 alone. Even so, his lax attitude toward forgeries inspired other artists to forge his work.

Honoré Daumier's case is more complex. Prints produced in his lifetime and by his authorized publisher are considered originals; all others are fakes. According to the Daumier Register, a digital catalogue raisonné for the artist, Daumier created 1,340 drawings in total, but there are another 1,562 drawings designated as 'after Daumier' and 'unconfirmed'. His oil paintings are even more off balance, with the Register claiming that he created 283 paintings but that 609 additional works are purported to be by the artist.

Auguste Rodin has been forged so often that some of his forgers are known by name. Among them is Odilon Roche, who collected drawings by the artist, and Ernest Durig, who claimed to be Rodin's last student and sold his forged drawings under the pretense that they were gifts from the master (including one that made it into the collection of the Metropolitan Museum of Art).