Google's obsession with data has defined a company that in eight short years has become one of the most successful businesses in modern times.

But take privacy into account, and Google's stated mission--as the dominant Internet search company, to make all the world's information accessible and usable--can have unsettling overtones. Google's success stems from its growing ability to fine-tune advertising pitches to users, thanks to its intimate knowledge of their online behavior. And for some that sounds scary.

In an age of growing concern over privacy, people worry about the government looking at their phone records. They fear having their medical records fall into public view or their library borrowing habits reviewed by federal agents.

Then there's the privacy showstopper from earlier this year: AOL imprudently released the search histories of 657,426 users onto the Internet. The gush from AOL was the biggest warning yet that just a few clicks of a computer mouse are enough to dump vast amounts of private data into the public square.

"This was the first time users really realized, `Wow, people are really logging everything I do,'

" said Eric Jensen, who researches Internet search behavior at the Illinois Institute of Technology. "That was the big shock.''

Google's competitors have taken note. Microsoft and Yahoo are beginning to focus on privacy because it's an issue consumers care about. The rivals are refining their products and sharpening marketing messages to gain ground on Google, which commands nearly a 50 percent market share in search, according to Nielsen/NetRatings.

"This is going to play out in competition on the Internet," said Andrew Sherman, a lawyer who has written a soon-to-be-published book on Internet privacy and the law. "People might say, `I'm going to use Yahoo as opposed to Google because they protect my data better.' "

Microsoft has adopted a stringent privacy review policy for all products in development. The company is touting its privacy awareness on its new search engine, Windows Live.

Google is hustling to stay on top of the privacy issue too. The company, known for storing records of virtually all the activity on its many Web services, says the trove of data helps its products get better over time. Google's spelling function and even the quality of its search results depend on its use of user data. It also helps the company track fraud in the form of heavy clicking that distorts Google's search results and advertising.

Still, Google is refining its product development programs in reaction to the privacy debate. "The key thing we're trying to do now is to be aware of the privacy implications of everything we do," said Nicole Wong, Google's general counsel for products.

Even so, Google is finding that the privacy issue can sting it in unexpected ways.

Take the AOL leak. AOL took the heat. But it turns out another company actually conducted the searches. That was Google, which also set the clock to store users' search records through at least 2038. All searches on AOL are powered by Google.

Building the honey pot

When job seeker Todd Malicoat first posted his resume online a few years ago--complete with his name, address and phone number--he freely traded his privacy for a chance at a better job.

"I gave up on privacy a long time ago," said Malicoat, who now works in New York as a consultant, advising companies on Internet advertising.

People trade privacy for utility all the time. A consumer gives up personal information to get a credit card. Grocery shoppers swap their anonymity for "club card" discounts.

Yet, even now, people have their limits. The same user who posts intimate details on his MySpace page might not want his college grades posted. A rap fan in a Google discussion group might get creeped out when a Ludacris ad suddenly pops up. A worker searching for cancer treatments on WebMD might not want the boss to know.

Despite user concerns, the search companies keep pushing the limits on what they do with user data for one reason: profit potential.