Remember Napster? The free Internet file-sharing program that nearly brought the music industry to its knees before it was shut down in court six years ago?

Napster has now gone legit -- offering music downloads for a fee. But the issue of free copying of music and movies on the Internet hasn't gone away. There are dozens of successors to Napster now out there, and sales of iPod and MP3 players are booming -- much to the distress of moviemakers and musicians who say their work is being stolen.

Today the U.S. Supreme Court is set to hear oral arguments on what some experts are calling the most significant copyright issue since 1984, when the court decided that Sony's Betamax video tape recorder didn't violate copyright law.

"This case is the next generation of that same issue," said Michael Madison, associate professor of law at the University of Pittsburgh and an expert in intellectual property law. "If you have an innovative computer technology that makes it easy and cheap to consume and share copyrighted content, is it legal to distribute that technology?"

The stakes are huge, with billion-dollar industries pitted against each other.

In one corner: Metro-Goldwyn-Mayer Studios, backed by 28 of the world's most powerful movie studios and record companies.

In the other corner: Online file-sharing services Grokster and StreamCast Networks, which allow people to search computers of others and download music and movies free of charge. They are supported by Kazaa, Morpheus, Limewire and other popular file-sharing programs, along with technology and consumer electronics companies.

Grokster also is supported by a group of engineering and computer science professors from nine universities, including David J. Farber, distinguished career professor of Computer Science and Public Policy at Carnegie Mellon. In a brief submitted to the court, they argued that a ruling against file sharing could chill innovation in computers and on the Internet.

At issue is an appeal of a 2003 ruling by a federal judge, later upheld by an appeals court, saying Grokster and other file-sharing programs could not be held liable for copyright violations. Citing the Supreme Court's 1984 ruling, U.S. District Judge Stephen Wilson said that file-sharing programs fell under the same copyright protections provided by the court in the Betamax decision. In that case, the Supreme Court agreed with Sony's contention that it didn't know what its video recorders were being used for once they left the store.

Grokster is using a similar argument. Unlike Napster, which provided computer servers that processed song searches, Grokster and its successors claim they provide a computer software program to users but don't know or control what people use it for.

The entertainment industry hopes to persuade the court otherwise, said Madison.

"The core of the case is what does Grokster know and when does it know it?" said Madison.

The lower court's decision prompted record labels and movie companies to change tactics and sue individual file sharers -- 9,900 so far, ranging from a 12-year-old girl to a 65-year-old grandmother. But in petitions before the Supreme Court, their lawyers argued that individual lawsuits can't possibly redress the "catastrophic, multi-billion-dollar harm" being inflicted by the millions of people using illegal file-sharing programs.

If the nation's highest court finds against them, "Copyright will soon mean nothing on the Internet, and the incentives on which our copyright system rests will be imperiled," record and movie company lawyers claimed.

In turn, Grokster and the high-tech industry argue that any clamping down on file-sharing could stifle development of new technology -- even making the iPod illegal -- and trample free speech.

The court is expected to issue a ruling before it recesses in June.