Ad Tracking: How Advertisers Follow You Across the Internet — and How to Stop It

You look at a pair of shoes on a retailer's website. For the next two weeks, those exact shoes follow you to news sites, social media, recipe blogs, and weather apps. This is retargeting — one of the most visible forms of cross-site ad tracking — and it's powered by an infrastructure of third-party trackers, tracking pixels, and data brokers that most users never see. Understanding how it works is the first step to stopping it.

The core mechanism is the third-party cookie. When you visit the shoe retailer's site, it loads resources from advertising networks like Google Ads, Facebook Pixel, and dozens of smaller ad tech companies. Each of these networks sets a cookie in your browser with a unique identifier. When you visit another site that uses the same ad network, the network recognizes your identifier and knows you're the same person who visited the shoe site — and serves you the shoe ad.

Tracking pixels are a related technology. A tracking pixel is a 1x1 transparent image embedded in a webpage or email. When your browser loads the pixel, it sends a request to the tracker's server — revealing your IP address, browser type, and the fact that you loaded that page or opened that email. Email tracking pixels are particularly invasive: they tell senders exactly when you opened their email, how many times, and from what location. Most email clients now block tracking pixels by default, but many still load them.

The third-party cookie is being phased out. Google announced plans to deprecate third-party cookies in Chrome, though the timeline has been repeatedly delayed. Safari and Firefox already block third-party cookies by default. In response, the ad industry has developed alternative tracking methods: first-party data (retailers collecting your data directly and sharing it with ad networks); fingerprinting (described in our browser fingerprinting article); and Google's Privacy Sandbox, which attempts to enable targeted advertising without individual-level tracking — though privacy advocates have criticized it as inadequate.

The most effective tool against ad tracking is uBlock Origin — a free, open-source browser extension available for Chrome, Firefox, Edge, and Safari. It blocks third-party trackers, tracking pixels, and ad scripts at the network level, preventing them from loading at all. Unlike ad blockers that hide ads after loading them, uBlock Origin blocks the network requests entirely — which also speeds up page loading significantly. Studies have found that uBlock Origin blocks over 90% of tracking requests on typical websites.

A complete anti-tracking setup: use Firefox or Brave as your primary browser (both have strong built-in tracking protection); install uBlock Origin with the default filter lists plus the EasyPrivacy list; enable your browser's built-in enhanced tracking protection; use a private search engine (DuckDuckGo or Brave Search) to avoid search-based profiling; and consider a DNS-level blocker like NextDNS or Pi-hole for network-wide tracker blocking. This combination blocks the vast majority of cross-site tracking without significantly impacting your browsing experience.