Illustration of a mask over a laptop screen to represent incognito mode in a browser.

You don’t have to be a computer whiz to grasp the value of private web browsing. At a time when consumers are worried about the data Facebook and others have assembled on their digital lives, it’s nice to have a browser tool that conceals some online activity.

All of today’s major web browsers—Chrome, Edge, Firefox, and Safari—offer a feature that provides a private browsing window and deletes the browsing history on your computer after you close it. (To open a private window, go to the File menu and look near the New Window option.)

These windows can help reduce the amount of information collected on you by retailers and advertising companies. They have other smart uses, too.

However, recent research (PDF) indicates that many people overestimate the protection provided.

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More than half of 460 people surveyed by University of Chicago researchers thought an incognito window would block Google from recording their search history even if they were logged into their Google account. More than 40 percent of respondents believed the tool would hide their location from websites they visited. And more than one-third believed incognito mode would shield their web browsing from an employer.

None of that is true.

“Private browsing mode does some useful things, but you’re absolutely not anonymous, you’re not ‘incognito,’ and your secrets are not necessarily safe” from hackers or marketers, says Blase Ur, an assistant professor of computer science at the University of Chicago who conducted the research. “You should still browse like people are watching.”

Here’s what private browsing really does—and what it doesn’t do.

How Does Incognito Mode Work?

When you browse the web in a regular, nonincognito window, the browser stores the URL, or web address, of every page you visit and keeps that information even after you close the window you’re in. That makes it easier for you to retrace your steps and find the same web pages again sometime later.

The browser also stores cookies, which are little files that websites and advertisers embed in websites. Next time your browser loads a page with elements from a company’s servers, the information is sent back. Cookies have lots of functions, such as letting you go to password-protected sites without logging in each time and keeping track of what you place in a shopping cart. They also let big advertising companies, such as Google’s DoubleClick, track you from site to site across the web.

Private windows act differently.

If you’re using incognito mode, “At the end of each session your cookies go away and you get a whole new set the next time you start,” Ur says.

The most obvious change you’ll notice after a privacy browsing session is that it doesn’t show up under the History tab in your browser. But you may also notice less tracking from advertisers. If you search for a product—blenders, say—in a private window, you’re not as likely to start seeing cooking supplies show up in web ads over the next few days.

Firefox adds a layer of tracking protection to its private browsing mode. This helps protect against a technique known as fingerprinting, in which data collectors track you around the web by comparing variables such as which browser version and operating system you’re using, which graphics card you have installed, and your IP address.

Why Is Incognito Mode Useful?

“If you just want a brand new session that doesn’t remember anything about who you are, then incognito mode will work really well for that,” says Jeremy Tillman, product director at Ghostery, one of several browser extensions that block web tracking.

Let’s say you’re shopping for a gift for your spouse on the family laptop—maybe a day pass for a local spa. Using incognito mode will prevent anyone else who might use that laptop from seeing that you searched the likes of Google and Yelp for “best spas near me.” And they won’t start seeing spa ads popping up over the following few days.

“Nothing spoils a birthday surprise quite like a targeted ad,” says Robert Richter, program manager for privacy and security testing at Consumer Reports.

The same goes if you wanted to watch one quick YouTube video about a celebrity gossip item or World Cup highlights without then being bombarded with related videos the next time you log into the site. An incognito window will keep that from happening.

And, Richter says, incognito mode has “espionage light” uses: If you want to read someone’s LinkedIn page without them knowing, you can employ an incognito window. 

Incognito mode could also come in handy when you’re visiting a friend and want to quickly check your email on his computer without opening his email account. Simply launch an incognito window, sift through your inbox, then close the window.

Consumers who print web-based documents using a public computer at a library or an office supply store may also want to use incognito mode because it will erase any personal data, such as Gmail usernames and passwords, when you close the window.

Don’t let incognito mode lull you into a false sense of security, though. Logging into personal accounts from a public computer—or even a friend’s—is always a more risky endeavor than your living room. Remember: Guard your passwords, and close that window when you’re done.

What Doesn't It Do?

Once you close an incognito window, most of the data about your web session will be deleted, “but only the pieces that were stored on your own computer,” Richter says. “The data stored on company servers as a result of your online activity is another story altogether.”

A private browsing window can’t erase the records of your visit from a website’s servers, or from any networks you went through to get to a site. If you’re on your employer’s WiFi network, your company will know which sites you visited, just as though you weren’t using a private window.

If a site isn’t safe for work, it’s not safe for work in incognito mode, either.

Incognito mode also doesn’t do anything to protect you from malware—for that, you should take the usual steps of ensuring that your software is fully updated, that you’re running trusted antivirus software from a company such as Avira or Symantec, and that you scrutinize the files you download.

And remember that any bookmarks you make or files you download while in incognito mode will persist after you close the browser.