HE Sims Online is a clean, well-lighted corner of the Internet where people work to build an elaborately decorated, chat-filled virtual world. But if playing by the rules in this realm isn't entertaining enough, there are after-hours joints where rogues and grifters gather to swap schemes for gaming the game and growing rich.

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The chatter at TSOExtreme.com, for example, is a mix of simple tips for guiding the characters known as Sims and elaborate strategies for earning millions of the online currency known as simoleans. Recently much of the talk has centered on using extra software, known as a bot, to automate the most tiresome clicking so players can rack up hundreds of thousands of simoleans in their sleep.

One of the players engaging in this automated counterfeiting, a 29-year-old financial planner from Texas, said he did so without apology (although he did not want to be identified by name). "I think the bots actually level the playing field for people who have day jobs," he said. "When I play an online game, I can't be the best because there are some college kids out there spending 14 hours a day."

Web sites like TSOExtreme.com are a challenge for the rapidly growing world of interactive games. While breaking the rules or using secret "cheat codes" has always been an accepted, even treasured part of single-player games, new online games match competitors, often strangers, remotely, which changes the dynamic. No one likes to lose unfairly, and those who play by the rules often struggle against schemers who believe that all is fair in love and simulated war.

For their part, many of the cheats say that bending the game's rules is part of the fun. It is only a game, and when it becomes boring it is time to turn to the greater game of beating the system, they argue.

Brian Reynolds, a designer of a new online game, Rise of Nations, likes to joke that he was "the guy who put 'Cheat' on the main menu" when he developed games like Civilization II. A player could use the menu at any time to create new assets like warriors or defenses for a city.

In his new game, however, in which players meet and battle for ratings over the Internet, that option is gone. Mr. Reynolds and his team try to ensure that people who buy the game have a pleasant and balanced experience when battling others to dominate a virtual world. They fear that people would stop playing if those who cheated held all the power.

Haden Blackman, the producer at LucasArts responsible for Star Wars Galaxies, an online game now being tested by 5,000 users, said that preventing cheating was one of the biggest challenges of creating a virtual world.

One lesson the game industry learned the hard way is that dedicated cheats will rewrite software to give themselves an advantage. "There are a lot of great ideas we come up with and skip because there's going to be 1 percent who will abuse them," Mr. Blackman said.

Designers of the new Star Wars game initially planned to let players communicate in strange languages that would be translated by other players' computers, he said. But the developers soon realized that cheats would find a way to break into the hidden dictionary, gaining the ability to speak the various languages and negotiate with aliens from other planets - a skill that would normally develop only over time.

Bots like the ones discussed on TSOExtreme.com are just the beginning. Some players of games with a shooter, like Quake or Counter-Strike, have automated aiming tools that target an opponent more rapidly than the quickest of fingers.

Others reprogram their video cards to hide the elaborate textured walls in a game. All that is left is a wire-frame outline, allowing a player to see through walls and track those hiding behind them.

All of these techniques depend on users' having full control of the software running on their home machines. Adept programmers can rewrite the game or insert new instructions. The other players can either play fair or join the arms race.

The game Rise of Nations challenges a player to take a civilization from stone axes to nuclear weapons. The biggest worry of its designer, Tim Train, is not so much tricks that let players triple their bankroll with a single click, as ones that reveal hidden information or parts of a map.

"We use a simultaneous simulation on each player's machine," he explained. "If your wealth is suddenly increased by 100 times, the other computers notice it and quit."

To prevent people from poking around the computer memory in search of information about the location of hidden objects, the game encrypts all communications and stores data in different places every time users play.

Some software makers are working on more aggressive solutions. Tony Ray, the president of the Houston-based company Even Balance, distributes a free product called Punkbusters that acts as a virus detector by looking for modifications on every player's machine. Game companies are paying for its development in the hope of keeping the games fair. Software installed on every player's machine watches for cheating while periodically filing reports to other players.

"When QuakeWorld came out online, the community was huge and teeming with people," said Mr. Ray, referring to a first-person maze game that was popular in the mid-1990's. "There was serious competition and an enormous amount of online status. Then the cheats showed up, and almost overnight it went from something that was a hugely popular community into something that was a wasteland.''

"All of the major developers were saying that they could do nothing to fight cheating because they couldn't control what went on in people's computers," he said. "The whole landscape of online gaming changed when we proved cheating could be fought effectively."

Mr. Ray's job is not easy. Every day he monitors discussion sites where cheats exchange notes and software. If a new tool emerges, he adds it to the list of unacceptable software. The cheats, of course, look for ways to keep their software off his list, and the larger game continues.

Tools like Punkbusters can only detect active reprogramming, not ways in which players abuse loopholes. (Game players often call these "exploits" to distinguish them from outright cheating.)

"Where we run into the gray area is when people do new things in games with the tools we've given them," Mr. Blackman said. "They're just using them in ways that we never expected."

Spencer Armstrong, a game tester in Calgary, Alberta, said he once found that a glitch in a virtual world called Neocron let his shots pass through a tree that blocked return fire. He recounted with a mixture of pride and chagrin that he killed some monsters to run up his score. Only after "taking full advantage" of the situation did he report the bug to the designers of the game, for which he was a pre-release "beta" tester. "During a beta, I play a little fast and loose," he said,.

But outside his circle of close friends, he said, he would never use such a trick. "If you're playing Counter-Strike online competitively against people you haven't met, cheating's wrong," he said. "It's as wrong as blood doping or taking steroids. But if you're playing a bit more for fun, just to explore, and you're playing with the game, then why not cheat?"

Gordon Walton, the executive producer for The Sims Online, said his staff monitors the state of the game, looking for anomalies. They also watch Internet activity and sites like TSOExtreme.com for new techniques for cheating. "If something goofy was going on, we would see it in 2.4 seconds." he said.

This policing, however, is never perfect. "I've never seen more than a tiny fraction of people cheat, but when they do, it can become a tactic," he said. "It's like how everyone can go five miles over the speed limit, because that's how it's enforced. If you leave a cheat long enough, it becomes part of the culture of the game."

Deciding when to step in and reprogram the game is a challenge for designers. Mr. Reynolds, for instance, said it was hard to outlaw a technique that was permitted by the game's logic.

"It may be fair when the game first comes out, but we still have to preserve the game itself," he said. "We'll start to patch it when it destroys the balance of the game."

Some players are still saddened that Electronic Arts, the publisher of The Sims Online, closed a loophole in the game that showered simoleans on anyone who stepped backward immediately after breaking a virtual piñata at an online party.

The worst nightmare for designers are tactics that give players unbeatable power, eliminating the pleasure of watching a game unfold. Even when a technique breaks no rules, balance can sometimes be restored only by banning it. Mr. Armstrong, for instance, was simply exploiting a loophole by shooting through trees. But if everyone did so, the challenge of the game would disappear.

Mr. Blackman said his team would pay extra attention to the economy in Star Wars Galaxies because designers have built in unparalleled freedom for players to create objects and sell them. In theory, this should give players many options and strategies to explore, but it could also lead to players' gaining monopolies. "I'm sure that six months after launch we're going to have plenty of stories," he said.

Sometimes the lines between the players and their game roles blur so that it is difficult to define what is fair. Star Wars Galaxies encourages players to adopt a persona from the "Star Wars" milieu, a world in which not everyone plays by the rules.

Asked how Han Solo, one of his favorite characters, would play, Mr. Blackman laughed and said he "would use any advantage he could get."

"I'm that way," he said. "If you give me an advantage in the game, I'm going to use it. We want to have some things for the power gamer to discover, but there can't be so many that it unbalances the game."