Here you are, one of the millions of Americans who like to play casual games on the Internet, ready to log on for some fun.

So you go to a gaming Web site and try to ignore the ads on the page. Then you wait as the game loads and a 10-second advertisement covers your computer screen.

Ah, the start button.

Now you can play, but don't expect the virtual escape to guarantee relief from marketers' attempts to get in front of your eyes.

On Monday, online game provider Shockwave.com began offering advertisers a way to insert ads within the games themselves.

While it's believed to be the first such invasion in Web-based games, it's only one of a growing number of venues advertisers are using to reach its shifting and fleeting audiences.

The traditional pillars of advertising in print and television media have eroded in recent years as people — especially the elusive demographic of young men — have instead spent more time on video games and on the Internet.

Hence, the ad creep, whether loudly from the walls of sports fields, subtly from the strategic product placements within films, or annoyingly from the pop-up ads all over the Internet.

There's no respite even when people use their TiVo digital video recorders to skip TV commercials: Earlier this year, banner-like ads started appearing during the fast forwarding.

Ads are also showing up in console video games.

It was only a matter of time then that in-game advertisements would arrive in the world of casual Web-based games. The often addictive genre of action, puzzle, and card games attracted nearly 56 million unique visitors in September, according to comScore Media Metrix, a research firm.

Already, advertising revenue from online games, including the more hard-core multiplayer games, is projected to grow to $1.1 billion by 2008, up from between $450 million and $550 million last year, according to the Yankee Group research firm.

Shockwave.com, a division of San Francisco-based AtomShockwave Corp., wants to capitalize on the growing migration of advertising dollars to the Internet.

"There's such a huge demand right now from brand advertisers," said Dave Williams, chief marketing officer of AtomShockwave. "And this is a huge audience, and an engaged audience."

Shockwave.com hosts more than 200 games and claims its 20 million visitors to the site last month played more than 25 million game sessions.

The advertising network to be launched Monday will allow marketers to insert their images or brand names inside the games. They'll be able to track the "impressions" or viewing times each ad gets — a key advertising metric — as well as tailor their ads to geographic markets.

SBC Communications Inc., Sprint Nextel Corp. and Sony Corp.'s Sony Pictures are among the first companies planning to use Shockwave.com's new advertising feature.

Shockwave.com plans to start ad insertions with action games, where the landscape, say of a racing game, or sport, lends itself to billboard-like advertisements.

For instance, in the game called "SWITCH Wakeboarding," players will soon see bright yellow Sprint Nextel ads interspersed on ramps as they buzz around the lake doing tricks.

In the game, which usually lasts about 15 minutes, a player might see ads as many as 25 times, Williams said.

Ad images will generally last from three to seven seconds in action games, and perhaps longer in other games where an ad can be displayed, say, on the hood of a car, instead of a passing object, Williams said.

Shockwave.com plans to later introduce ads in mind and puzzle games, too, but only if they could somehow be incorporated into the design without interfering with the game play.

Players should never see an ad that will pop up and block their views as they're maneuvering their marbles, tiles, or jewels in a puzzle game, Williams said.

"Consumers are not screaming for more ads," he said, "And we want to make sure that as we roll this out, that the places where you'll see the ads will be where you would expect to see them in the real world as well."