AKRON, Ohio — On July 10, 2004, at 3:20 p.m., W. Logan Fry had an epiphany in his Richfield, Ohio, farmhouse.

"Create content for cell phones. Cut it away from the computer screen," he scribbled in Smart Mobs, a book he was reading about how technology changes the way people interact with the world.

Fry, director of the Web-based Digital Museum of Modern Art, thought the concept of targeting people through their cell phones, otherwise known as mobile marketing, was brilliant.

He saw the cell phone as another — perhaps better — way to show off the paintings, poetry and sculptures made by local artists.

"It's a museum in your pocket," said Fry, who runs the museum from his family's farm.

Someone who surfs to www.dmoma.org to see full-size images of art on a home PC also might surf to www.dmoma .org/dex.html to see scaled down images on a cell phone. And vice versa.

Just 20 years ago, cell phones were known as "the brick." Ten years ago, they were a luxury item. Today, they are tiny, sophisticated and everywhere. In the United States alone, about 180 million people own cell phones. And of that number, more than half are sending text messages, surfing the Web, downloading ring tones and playing games.

That broad and active audience makes cell phones a huge target for companies looking to promote their brands with related games, ring tones and other content.

This may sound like a nightmare for the millions of Americans worried about being bombarded with marketing solicitations, no matter where they are. But as it stands now, marketers can't hit you up with text messages to buy Viagra unless you ask them to.

"The wireless device is a sanctuary of privacy," said Roger Entner, vice president of wireless telecom for the tech consulting firm Ovum. Nevertheless, the industry is still in its infancy.

The Mobile Marketing Association counts any form of mobile content — ring tones to video clips to news alerts — used to promote a product as part of the industry. Under those rules, the industry will rack up about $350 million worth of sales this year. But by some estimates, it could grow to billions of dollars by next year.

It will become typical to see someone playing a game tied to the next big summer blockbuster, or hear a jingle for some fast-food chain as someone's ringer.

"What's different about the cell phone is, more than any other device in history, people carry them on their person most of the day," said Bryan Biniak, senior vice president and general manager of AGmobile, the wireless division of Cleveland-based American Greetings. " ... People go to bed with their cell phones. They take them into the bathroom. Because of that one-on-one relationship, it's going to be the most powerful marketing tool in history."

For all the promise of mobile marketing, there's one big reality check: the industry is controlled.

Like Fry, anyone can post something on the Web for cell-phone subscribers to see. But the owner of a mom-and-pop diner can't just create a ring tone out of their advertising jingle, call up Sprint PCS and say, "Put this on your network so people can download it."

Companies that want to get their products in front of cell-phone subscribers must go through middleman companies known as aggregators. These companies have agreements with all the major cell-phone carriers, such as Cingular, Verizon, Sprint and T-Mobile. They help companies develop marketing campaigns and content for cell phones.

Aggregators wield a lot of power, but there are only a few of them, namely m-Qube, Mobliss, Enpocket, Air2Web and AGmobile.

It's easy to create a ring tone or an image for subscribers to download, but it's not so easy to distribute either to the masses. Every cell-phone model and carrier has different technical requirements.

But putting that bottleneck of aggregators aside, consumers have the ultimate power in mobile marketing.

They decide whether they want to opt in or opt out.

So, for instance, a television commercial for Pringles could tell viewers to send a text message to 1111 if they are interested in playing a Pringles trivia game. A subscriber would get access to a Web site with the game. Once there, the subscriber could buy a ring tone or an image to use as wallpaper on his phone. But Pringles also could send the subscriber text messages. Eventually, the subscriber could opt out and the messages would stop, along with his access to the site.

Cell phones by the numbers