Imagine that when you walk into a mall, someone straps a device to your arm that tracks your shopping habits and other vital information. Each store you enter uses the device to learn about your purchasing behavior -- for example, what items you examined and rejected -- and maybe even to obtain personal information about how old you are and where you live. Would you accept this sort of shopping experience? If you use the Internet, it's a moot question; you already are being tracked this way.

Most online shops already use a tracking technology known as "cookies." This is computer slang for data that is stored on your hard disk so that a website will recognize you next time you arrive. You supply the information in each cookie -- usually by completing a registration form at the commerce site. The site then sends this information back to your computer for storage and later retrieval.

The information placed in a cookie is not only useful in the context of e-commerce. It can also enable you to receive a customized home page that, for example, supplies entertainment news, but not sports. Finally, cookies provide marketing information; they can track the ads you click on in order to provide you with similar banner ads in the future.

If you want to see what information is stored in your cookie file, use a text editor or a word processor to open a file called cookies.txt or MagicCookie in your browser's folder or directory. Or visit http://www.cookiecentral.com/stopcm.htm, a Web page that allows you to view the contents of your cookie files.

The tracking information that DoubleClick and others collect is merged with information voluntarily supplied by consumers to form profiles that are used to target advertising. Consumer watchdog groups object to this compiling and use of personal data obtained by using cookies as tracking devices. Consumer complaints have also spurred the Federal Trade Commission to investigate the privacy concerns arising from the use of cookies.

So far, no laws ban or limit the use of cookies when gathering information about adults. But Section 312 of the Children's Online Privacy Protection Act (http://www.gseis.ucla.edu/iclp/coppa.htm) prohibits the use of cookies and other passive tracking devices for the purposes of collecting or maintaining personal information from a child unless parental consent has been given.

Currently, cookie-using firms such as DoubleClick take in data unless consumers notify them they don't want it used. For example, to prevent DoubleClick from gathering information, you must go to their site and "opt out" of both "email cookies" and "ad cookies." The problem is that most consumers, who don't fully understand the extent to which their privacy is at risk, don't know how to opt out. That's why many privacy groups favor legislation that requires sites to get consumer permission (or "opt in") before collecting or using the data gathered by cookies.