In 1993, the dawn of the Internet age, the liberating anonymity of the online world was captured in a well-known New Yorker cartoon. One dog, sitting at a computer, tells another: “On the Internet, nobody knows you’re a dog.” Fifteen years later, that anonymity is gone.

It’s not paranoia: they really are spying on you.

Technology companies have long used “cookies,” little bits of tracking software slipped onto your computer, and other means, to record the Web sites you visit, the ads you click on, even the words you enter in search engines  information that some hold onto forever. They’re not telling you they’re doing it, and they’re not asking permission. Internet service providers are now getting into the act. Because they control your connection, they can keep track of everything you do online, and there have been reports that I.S.P.’s may have started to sell the information they collect.

The driving force behind this prying is commerce. The big growth area in online advertising right now is “behavioral targeting.” Web sites can charge a premium if they are able to tell the maker of an expensive sports car that its ads will appear on Web pages clicked on by upper-income, middle-aged men.

The information, however, gets a lot more specific than age and gender  and more sensitive. Tech companies can keep track of when a particular Internet user looks up Alcoholics Anonymous meetings, visits adult Web sites, buys cancer drugs online or participates in anti-government discussion groups.