With downloadable "CSI" episodes coming to local Comcast customers for 99 cents a pop in January, the long-standing rules of television programming are being thrown out the window.

Instead of network executives deciding what shows fill the prime-time slots, viewers are getting to be their own programmers, watching shows at their convenience.

"We're crossing over from 50 years of a mass media world dominated by television to a brave new world of personal media," said Paul Saffo, the director of the Palo Alto, Calif., think tank Institute For The Future.

TV will still be a major part of people's lives -- in the past year, Americans were tuned in for a record eight hours and 11 minutes a day, according to ratings service Nielsen -- but the way they experience it is changing, right before our eyes.

"Mass media and television delivered the world to our living room, but all we could do is press our nose against the screen and watch," Saffo said. "Personal media, in contrast, is very much a two-way trip. We can more easily talk back."

Some viewers already watch shows on their own terms and times, aided by an explosion of technology. VCRs are heading toward extinction as TiVos and other digital video recorders allow viewers to watch shows whenever they want and to skip commercials. They also can trade shows online, using BitTorrent cooperative distribution software.

Networks saw that fast-moving train rushing by them, explaining why they finally agreed to jump onto the 99-cent download model.

ABC started it Oct. 12, allowing Apple to offer $1.99 downloaded episodes of "Lost" and "Desperate Housewives" for use on its new video iPods.

Last week, the two other major networks followed suit: CBS allowed Comcast to offer digital cable users shows, including "CSI" and "Survivor," while NBC agreed with DirectTV satellite on such shows as "Law & Order: SVU" and "The Office."

The CBS and NBC shows will cost 99 cents each, the same amount iTunes charges for music at its popular download site.

A flood of similar deals are sure to follow, ultimately dwarfing last week's announcements, making more and more shows available, through varied platforms.

"The iPod video player doesn't matter. Downloading episodes of 'Lost' and 'Desperate Housewives' to computers barely matters. What does matter is the crack in the traditional television business model," analysts at Cambridge, Mass., tech firm Forrester Research, said Monday.

Some networks already have skipped the traditional television model and started shipping shows, some of which are produced for online audiences only, directly to the Web. With the growth of broadband, up from 25.3 million households in 2003 to an estimated 42.3 million this year, watching the shows on computers has become easier.

CBS streams episodes of its sci-fi drama "Threshold" after they air along with Web-only extras, including a blog by show writers. Web sites for MTV and Comedy Central, both owned by Viacom, as is CBS, also feature online extras, which viewers can e-mail to friends, sparking virtual watercooler talk and giving the programs second life online.

Scripps Networks is rolling out some 10 broadband-only TV channels, as well as a Web-only cooking show on the Food Network's site. Disney is unveiling a Web channel featuring clips by its cartoon characters that is aimed, of course, at kids.

With few exceptions, such as an ABC News online package that costs $39.95 annually, the Web content is free. In most cases, viewers have to sit through commercials to watch the shows, making the computer-bound viewers a viable new customer base for advertisers and helping to fuel the move to downloadable shows.

As more network programming becomes available for on-demand use, analysts expect that the 99-cent charges for watching "Survivor" or other shows will be dropped, and that they, too, will be offered for free, as long as the monthly cable bill is paid, that is.

Comcast, the nation's largest cable company, hopes that is the case.

"It's a bit of a test for Viacom to see if this makes sense. They actually are charging us for these shows and we are passing the cost on to our customers," Doug Sansom, Comcast's senior vice president for the Western Atlantic region, said of their 99-cent download deal. "We prefer to give our content away for free."

And how could they offer shows for free? Just like broadcast television does, through ads.

"Obviously, our business is built on advertising revenue," Mr. Sansom said. "We've got to keep advertising involved in these programs or, frankly, there's just not enough money out there to continue to pay for great content."

Brad Adgate, senior vice president of research at Horizon Media in New York, said these new technologies open up more opportunities for advertisers to reach consumers.

"It just creates more fractionalization of the viewing source that has been around for the past 25 years," Mr. Adgate said. "There are issues, of course. One is attentiveness and who's going to measure this stuff, but, that aside, I think it's going to be more hyper-targeting. You're going to see more relevant commercials to consumers that will get them more engaged in the products."

Mr. Adgate doesn't expect the 30-second commercial to disappear overnight, but it's possible broadcast network ad sales have peaked. In May, networks saw modest gains for sales of advertising purchased before the start of the TV season, an indication of the coming changes, Mr. Adgate said. "They didn't get the humongous increases in ad rates, and part of that had to be coming to grips with the other opportunities advertisers have to spend their dollars."

So what does the future hold?

Just like the fights over copyrighted music, there will be long fights over free digital video content. Some will insist on watching shows without fees or ads, while others will press hard, probably through the courts, to get paid.

Networks will have to negotiate deals with their affiliates, which are sure to want a slice of on-demand revenue. (It's important to note that CBS's deal with Comcast is applicable only in markets where CBS owns stations, limiting the objections from non-owned-and-operated affiliates.)

Online television will grow, and new networks will grow up around it. The crossbreeding has started with a deal that will let TiVo users get Yahoo! content through their computers, plus Google's nascent attempts to allow searches for TV programming, which probably would include search-based ads, just like their regular searches.

Programming itself has started changing to fit different technologies, as shown by the Web-only shows some networks are doing. That will continue, along with shows produced with television sets in mind, some for personal computers and others for mobile use.

Screens on iPods and other mobile players are small and largely used during commuting or other short bursts of attention, so programming for them will follow suit. People will use them to watch snappy music videos and eye-catching ads, not "The Sorrow and the Pity."

"Traditional TV does not go away. We still have two screens in our lives and we've just added a third," said Mr. Saffo, the Institute For The Future director.

"No old media form ever disappears. They get reinvented into a new purpose. TV is about to go through a profound reinvention."

Q: Will on demand programming have commercials?

A: ABC's shows available through iTunes do not, nor do NBC's on DirecTV. CBS' shows via Comcast VOD will have commercials.

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Q: Paying to watch otherwise free TV shows on our own schedule: Is this what the TV on DVD revolution has wrought?

A: Yes. Sales of TV shows on bulky VHS tapes never amounted to much, but studios were surprised to discover an untapped revenue stream with TV shows on DVD.

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Q: But will VOD cannibalize TV on DVD sales?

A: "I think they're going to be very careful to not cannibalize," said Scott Hettrick, editor in chief of DVD Exclusive magazine. "It definitely has the potential to, and not just TV on DVD, but TV in general. There's a lot of concern among the TV executives right now. Everybody is looking for new ways to grow revenue."

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Q: Will I own the on demand shows, like I do my DVDs?

A: Not for now. They are more like rentals. Except for reality shows, which don't do well in repeats, most CBS and NBC shows will be available for a week or so, until new episodes air. You will own ABC shows purchased through iTunes, but they can be played only on your computer or iPod. They cannot legally be burned onto discs.

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Q: Is America at the forefront of on demand offerings?

A: Hardly. In the United Kingdom, The BBC plans to make all its current programs available online next year.

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Q: What kind of programming is available legally online in America?

A: It's spotty. CBS.com is streaming episodes of "Threshold" after they air on TV. Google briefly had the first episode of UPN's "Everybody Hates Chris" available for streaming and Yahoo! streamed the premiere of The WB's "Supernatural" before it aired on TV. This week, MySpace.com announced it would make an episode of the Fox series "Kitchen Confidential" available online before it airs on TV. Both Fox and MySpace are owned by News Corp.

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Q: What is BitTorrent?

A: It's a peer-to-peer file distribution application that allows the computer-savvy to download files, including TV shows, from multiple sources, requiring less bandwidth than traditional downloads. For details: www.bittorrent.com.