NEW YORK (CNNMoney.com) -- The calendar says August. But for big TV networks, the leaves might as well have already changed colors.

Fox, which finished in first place among the advertiser-friendly 18-49-year-old demographic last season for the second year in a row, will officially kick off the fall TV season when it brings back its hit series "Prison Break" on August 21 and launches the new show "Vanished."

NBC's "Studio 60 on the Sunset Strip", a "dramedy" about what goes on behind the scenes at a sketch comedy show is getting a lot of favorable buzz for the Peacock Network...

...and so is NBC's "Heroes" about average people with super powers.

Media buyers expect CBS' "Shark," starring James Woods, to be one of the bigger new shows of the fall.

ABC hopes that fans of "Lost" will stick around for "The Nine," a new show that follows the lives of nine strangers after a bank robbery.

It will be an interesting year for all the major networks, one that promises much change. In addition to News Corp.'s (Charts) Fox, CBS (Charts), Walt Disney (Charts)-owned ABC and GE's (Charts) NBC, viewers and advertisers will be faced with two new networks.

The CW, created from the merger of CBS's UPN and Time Warner's WB, debuts in September. (Time Warner (Charts) also owns CNNMoney.com.) And MyNetworkTV, also owned by News Corp., launches in September and will only air English-language telenovelas, serialized dramas that run five nights a week.

With both networks aggressively targeting younger viewers, it remains to be seen how they will affect ratings at the Big Four.

"This is the first time two new networks have launched with a full prime time schedule," said Bill Carroll, vice president and director of programming for Katz Television Group, a media consulting firm.

Industry analysts also are wondering whether NBC, which has finished in fourth place with 18-49-year-olds for the past two years, will be able to emerge from the ratings doldrums.

Hopes are high since several new NBC shows, including Aaron Sorkin's "Studio 60 on the Sunset Strip," the Tina Fey comedy "30 Rock" and "Heroes" are receiving a lot of favorable advance notice from critics.

"NBC really needs this schedule to work. But they are getting a lot of buzz with new shows," said Chris Boothe, president and chief activation officer with Starcom USA, a media buying firm.

NBC also will be airing National Football League games on Sunday nights this fall. Media buyers say football should not only guarantee NBC big ratings on Sunday but should help NBC promote its other shows.

Looking at the other networks, CBS is expected to do well since it is returning most of its hits and is only launching four new shows. "Shark," a drama about an irascible attorney played by movie star James Woods, is expected to score big ratings since it will air at 10 PM on Thursdays, after CBS's powerhouse "CSI: Crime Scene Investigation."

But it will be interesting to see if the Eye Network can fend off a challenge from ABC on Thursday nights. ABC is moving one of its big hits, "Grey's Anatomy," to Thursday nights at 9, putting it up directly against "CSI."

Media buyers are mixed on how well ABC will do. Even though it has several big hits returning, such as "Grey's Anatomy," "Desperate Housewives" and "Lost," the network also has more new shows debuting than any other network.

"The Nine," a drama that will air after "Lost," and "Ugly Betty," a comedy that will run before "Grey's Anatomy," are getting positive attention.

As for Fox, most expect the network will be fine. It will air the baseball playoffs in October and has big hits such as "House" coming back as well as a highly touted comedy called "Til Death."

And even if ratings aren't spectacular during the first few months of the new season, most expect Fox to surge back to the top in January since that's when a certain talent show will return.

"I think more people will pay attention to Fox when 'American Idol' is back," said Carroll. That show's Tuesday night performance episode was the most-watched TV program last season according to Nielsen Media Research, and the Wednesday night results show came in second place.

Still, Kris Magel, senior vice president and national broadcast account director with ZenithOptimedia, another media buying firm, said advertisers really don't spend too much time worrying about which specific network is on top. They are more interested in how individual shows do.

"The network horse race doesn't do much for media buying. None of us could care less," he said. "Advertisers are looking more for programs that carry buzz. With 'Grey's Anatomy,' for example, you know when your commercial shows up, it will be more impactful."

Along those lines, this TV season promises to offer a lot of changes for advertisers that could affect the way commercials are bought and sold on television in the future.

Nielsen will begin publishing ratings for commercial time slots, a move that is expected to show that the audience advertisers are trying to reach on TV may not be as big as once thought, especially since more couch potatoes are using digital video recorders to fast-forward through ads.

There also has been talk of some prominent advertisers teaming up with eBay (Charts) to launch an auction system for buying commercial airtime.

In addition, network TV is no longer the only game in town for advertisers. There's cable to consider ... not to mention the Internet. Networks such as ABC and CBS are even offering free streams of some of their hit shows online with ads embedded in the online versions.

"It's pretty simple. If you look at how things are evolving, the way that consumers engage with media is changing," said Starcom's Boothe.

So with more options for viewers and, hence, advertisers, big companies are clearly more interested in ensuring that the money they spend on commercials will truly pay off.

Daniel Hassan, co-chief executive officer of Backchannelmedia, a marketing agency that specializes in direct-response marketing (ads featuring phone numbers or web site addresses for consumers to contact), said Fortune 500 executives he talks to "cringe" at the thought of committing so much money to TV shows months in advance since it's uncertain how effective the ads will be.

He speaks from personal experience as well. Hassan's father is Fred Hassan, the chief executive officer of pharmaceutical company Schering-Plough.

But Katz's Carroll said the so-called TV upfronts, the period during the spring when the bulk of television advertising for the fall season is bought, won't die anytime soon. He's not sure if an auction system for ads will work.

"The way we used to do business, someone sits at a desk and gets a phone call. They go have lunch and have some cocktails and sign a deal. We don't do it that way anymore," Carroll said. "Now, there are electronic invoices and qualitative analysis and product placements and sponsorships. It's more complex."

"So something as simplistic as an auction might not be the ultimate answer," he said.

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The reporter of this story owns shares of Time Warner through his company's 401(k) plan.