Years after rival web browsers made the move, Google Chrome on Jan. 4 will begin blocking websites from using third-party cookies, the easiest way to track our online behavior as we move around the web.

The browser will block third-party cookies for 1% of users on computers and Android phones, said Anthony Chavez, leader of Google's Privacy Sandbox project, in a blog post. Google will extend the block to all Chrome users by the end of 2024 under a schedule that has been pushed back several times in recent years.

The Chrome change, even though it so far only affects a small portion of people, is a momentous change for the web. Cookies, small text files that websites store on phones and PCs, have been used nearly since the dawn of the web, and ejecting them has been tough despite a growing effort to protect privacy online. Chrome is the dominant browser, accounting for 63% of web usage, according to analytics firm StatCounter.

Major browser competitors, including Apple Safari, Mozilla Firefox and Brave, began blocking cookies years ago, and Microsoft Edge offers the same with a "strict" privacy setting, but Google moved more slowly. It was more cautious about undermining the online advertising industry, which supports many websites as well as advertisers. And the UK's Competition and Markets Authority intervened in 2021 with concerns that Chrome blocking third-party cookies would give an unfair advantage to Google's advertising business by letting the company track behavior on its own websites without third-party cookies.

Cookies have plenty of benign uses, like remembering your language preferences, protecting against fraud or making it easier to return to a site without having to log in again. Many of those uses involve first-party cookies, though, not third-party cookies that can be set by advertisers showing ads or social networks adding share buttons. And worse things can happen than seeing an ad for a particular shoe on Amazon after you looked at the product elsewhere on the web.

"In the worst cases, third-party cookies are used to track users around the web, building up a detailed profile of them that could include not only interests but also deeply personal information such as gender, sexuality, religion, political affiliation, etc.," said Chris Mills, a technical writer who previously worked at Mozilla, in a post on the MDN site for web developers. "This information can be used to build creepy, invasive online experiences and is also sold to other third parties."

Without cookies, some have employed tracking technologies that are more surreptitious and harder to block, like fingerprinting that identifies characteristics of your computing device. Now Google and others are working on replacements for at least some of what cookies offered, for example, helping advertisers know if their ads have been seen. Finding a way to help advertisers while protecting privacy has been tough, but Google believes it's possible.

"As we work to make the web more private, we'll provide businesses with tools to succeed online so that high quality content remains freely accessible — whether that's news articles, videos, educational information, community sites or other forms of web content," Chavez said.

Google has worked to build new tools to substitute for third-party cookies. For example, a programming interface called Topics is designed to help with targeted advertising without tracking your website activity. But even when that's added to Chrome and other browsers like Edge based on its Chromium open-source foundation, other browsers like Safari and Firefox don't support it.

Correction, Dec. 15: This story initially misstated the time when Google will start blocking third-party cookies for 1% of users. It plans to start Jan. 4.