For more than 40 years, two well-known Picasso paintings, "Boy Leading a Horse" and "Le Moulin de la Galette," have been in the collections of the Museum of Modern Art and the Guggenheim Museum, respectively, in New York.

But who really owns them?

Officials at the museums were surprised last year when Julius H. Schoeps, a great-nephew of an earlier owner of the paintings, filed a lawsuit claiming the works had been sold under duress during the Nazi era and therefore belong to him. The case, one of many involving art lost or stolen during the World War II era, is still pending.

Both museums have denied the claims.

Some works of art are stolen and then simply vanish, permanently. Take the 13 works taken from Boston's Isabella Stewart Gardner Museum in 1990. Two men wearing fake mustaches removed art worth an estimated $300 million, including masterpieces by Rembrandt and Vermeer. None of works has been recovered. The ultimate art robbery

Sometimes stolen art appears on the market and is unwittingly bought by a collector.

"Many people think they are getting great artworks without proper research and understanding, and that can lead to deep disappointment and regret," says Thea Westreich of Thea Westreich Art Advisory Services in New York.

That can lead to serious disappointment for collectors and investors, especially those who pay top dollar and are investing in art in hopes of a big return.

As the Picasso cases suggest, transfers of ownership can be murky. Recently, there has been a surge of cases involving Nazi-era art. During the chaos of the war years, thousands of artworks were lost or went missing, many belonging to Jewish owners. A good number have resurfaced in the hands of new owners, including museums. Lack of proper records brings the new owners into conflict with the former owners or their heirs.

The Nazi era isn't the only source of such problems. Major museums such as the Getty in Los Angeles and the Metropolitan in New York built great collections of antiquities on acquisitions, some of which might have illegally left the countries of origin, including Greece and Italy. More recently, the war in Iraq led to the looting of national museums and archaeological sites and to the subsequent appearance of prized antiquities on the black market. Slide show: 5 global art heists

Perfectly innocent buyers can become victims. One recent case involved movie director Steven Spielberg.

In 1989 he bought a Norman Rockwell painting called "Russian Schoolroom" from a legitimate dealer. Last year he was informed that the painting, valued at an estimated $700,000 by the FBI, had been stolen from a Missouri art gallery years earlier. The link between the theft and Spielberg's later purchase of the painting was established by the FBI's art-crime team. The team estimates that about $6 billion in losses are incurred annually due to cultural property crime. How the FBI tracks big art crimes

Another risk: That Picasso might not really be a Picasso. It could be a fake or a case of mistaken attribution. A mistake can be uncovered when art historians and scholars discover new evidence or when a scientific test overturns a previous determination. A number of works once attributed to Rembrandt and Vermeer have met this fate.

So how can purchasers protect themselves?

Rule No. 1: Get the records. They establish provenance, a history of the work's ownership.

"You want to know who the prior owners were, going back as far as you can," says Sharon Flescher, the executive director of the International Foundation for Art Research, a not-for-profit organization in New York. The foundation deals with issues of attribution, authenticity, theft and looting of art. For instance, it investigated a work purported to be an early Jackson Pollock. It turned out to have been painted by his brother, Charles Pollock.

Rule No. 2: Opinions can be helpful, but you can't take them to the bank.

"Specialists occasionally disagree, especially over old masters," Flescher says. "Expert opinion can change as new scholarship becomes available."

Rule No. 3: Buy from reputable dealers and auction houses that strive to document and authenticate works passing through their hands.

Rule No. 4: Before putting down cash, check with stolen-art databases, such as The Art Loss Register, which lists about 170,000 items of lost or stolen art, or the FBI's National Stolen Art File. Do your homework by investigating museum exhibition catalogs and the catalogue raisonné, which lists all works attributed to a given artist.

Rule No. 5: Consider art title insurance. It's similar to title insurance on a house and is a new way to protect collectors. A Denver company called Aris, which deals with art ownership and art transactions, has been offering this insurance policy for the past year.

"Galleries say, 'Buy from me, I have done the due diligence,'" Aris President Judith Pearson says. "But the fact is, there is still risk out there."

The one-time average premium for Aris title insurance costs between 1% and 5% of the current value of the work. If an ownership claim is made against an artwork you possess and you lose, the policy reimburses you and pays legal costs.

Such coverage for a potential legal battle over a disputed work can come in handy because ownership cases can drag on. An established rule in the art world is that if a seller says something is by Picasso, that assurance is similar to a warranty. If it turns out not to be true, or if the authenticity is questioned, the warranty may have been breached. The buyer might have a solid claim against the seller in a civil suit, if the suit is filed within a state's statute of limitations -- usually four years from the date of purchase. After that, the buyer could be out of luck.

So it pays to do your homework before investing, because despite increased awareness over the past decade -- and new technology to track art -- lost and stolen art remains a major problem for collectors and art lovers worldwide.

"There are thousands of new works stolen each year around the world that we know about," says Flescher, of the International Foundation for Art Research. "The movement of stolen art knows no national boundaries."

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Published April 21, 2008

Produced by Elizabeth Daza/Graphics by Joe Farro