Think your favorite website isn’t tracking you? Think again. At least 75 percent of the 500 most popular sites—including Amazon, Facebook, and Google—use web tracking tools, according to a recent study by the University of Washington.

Companies collect your data largely through lines of code called cookies (see “How Targeted Ads Work,” below). Many websites implant cookies onto a computer’s hard drive the first time a user visits. 

Basic cookies can be useful. They enable sites to remember your log-in information, for instance. But so-called third-party cookies can track a user within a whole group of websites. So after you’ve been reading about sports on one site, you might see ads for sneakers on other, seemingly unrelated sites.

Think your favorite website isn’t tracking you? Think again. At least 75 percent of the 500 most popular sites use web tracking tools. That is according to a recent study by the University of Washington. These sites include Amazon, Facebook, and Google.

Companies collect your data largely through lines of code. They are called cookies (see  “How Targeted Ads Work,” below). Many websites implant cookies onto a computer’s hard drive the first time a user visits.

Basic cookies can be useful. They allow sites to remember your log-in information, for example. But so-called third-party cookies can track a user within a whole group of websites. If you read about sports on one site, for example, you might see ads for sneakers on other, seemingly unrelated sites.

Ad targeting is legal, with some exceptions. Federal laws bar companies from collecting online info from kids ages 12 and under. Teens have fewer protections—and they’re a very attractive market. 

Generation Z—the term for people now between the ages of 7 and 21—commands $44 billion in buying power, a recent study found. And most Gen Z-ers spend at least five hours online per day, which makes them especially vulnerable to ad targeting.

That’s why Facebook’s ad- targeting practices came under fire last spring. The Australian news- paper reported that Facebook employees had boasted to a potential advertiser that the company can pinpoint when its young users—including 1.9 million high school students—feel “overwhelmed,” “anxious,” “stressed,” or like “a failure.” 

Critics said it would be unethical for Facebook to sell such details to marketers, who could then advertise to teens based on their emotions. (The company says the data stayed anonymous and was never used for ads.)

It’s hard to prevent ad targeting entirely (see “Stop Following Me!,” below). Still, just being aware of the details you’re giving out when you go online is an important start, says Rachel Goodman, a staff attorney at the American Civil Liberties Union.  

“People need to have a better understanding of what the data is,” she says, “and where it’s going.” 

Ad targeting is legal. However, there are some exceptions. Federal laws ban companies from collecting online information from kids ages 12 and under. But teens have fewer protections. And they’re a very attractive market.

A recent study found that people between the ages of 7 and 21 command $44 billion in buying power. People in this age group are known as Generation Z. And most Gen Z-ers spend at least five hours online per day. That makes them especially vulnerable to ad targeting.

That’s why Facebook’s ad-targeting practices came under fire last spring. The Australian newspaper reported that Facebook employees had bragged to a possible advertiser. They claimed that the company can tell the emotions of its young users. These users include 1.9 million high school students. The employees said they can tell when the young users feel “overwhelmed,” “anxious,” “stressed,” or like “a failure.” 

Critics said it would be unethical for Facebook to sell such details to marketers. Marketers would then be able to advertise to teens based on their emotions. (The company says the data stayed anonymous and was never used for ads.)

It’s hard to prevent ad targeting entirely (see  “Stop Following Me!,” below). Still, just being aware of the details you’re giving out when you go online is an important start, says Rachel Goodman. She is a staff attorney at the American Civil Liberties Union.

“People need to have a better understanding of what the data is,” she says, “and where it’s going.”