Imagine a typical day online: you start by checking texts and emails, then scroll through Twitter on the bus and post a photo of your morning coffee to Instagram, tagging the coffee shop. At lunch, you check showtimes for a movie that a friend recommended on Facebook and look up prices for some new shoes you’ve had your eye on.

That’s only a few websites, but every time you’re online, you leave behind traces of your activity. Behind the scenes, a host of “third-party” companies – entities that are separate from the sites you visited – can track your activity and collect your data as you move through the Web. Later in the day you begin seeing recommended tweets about that movie, Web ads for those shoes and suggestions for coffees to try. It’s no coincidence, it’s data tracking at work. And it’s working on tracking your browser, apps and emails.

Not all data collection is bad. Websites often save your data to better personalize and improve your experiences with them. They use “first party” cookies – small data files placed in our browsers – to remember your language, layout preferences or the contents of a shopping cart.

Third parties, on the other hand, work with websites to insert additional tracking methods – such as their own cookies and Web beacons – to record what you read, click and visit online. This data collection, which is invisible to users, reveals more about you than where you’ve been. It creates a picture of everything about you from your preferences to your identity. Advertisers use that data to target you with ads and content across the Web and on your smartphone.

Online advertising is nearly inescapable, especially on social networks and media sites. There’s a logic to this: it is the main way that most online enterprises and publications sustain themselves. It supports a lot of the Web. Two of the five largest companies in the world– Facebook and Google – earn almost all of their revenue through advertising. News and entertainment media also rely on advertising to support journalism and content creation.

Not all online advertising is bad. On the up side, online tracking should deliver you more useful, relevant ads. On the down side, many advertisers don’t offer users real choice and control over what data is collected about them. Your online movements are sometimes collected by data brokers who are able to merge anonymized online data with personally identifiable information (information you may have volunteered on a form, in an app or that was collected offline) to build a surprisingly detailed profile of you.

As more advertising goes mobile, everyone should consider the data they share through their mobile devices. Especially as tracking goes far beyond attempts to sell to you. Banks can use it to determine your creditworthiness, and insurers can use it to assess your premiums. Researchers at the University of Washington have shown that mobile ad networks tracking capabilities could be manipulated for highly targeted surveillance.

To combat intrusive ads and deflect privacy-invading trackers (and not to mention boost speed and save data), more people are turning to private browsing and ad blocking techniques. According to a 2017 study by PageFair, a firm that helps companies recoup lost advertising revenue, 615 million devices deploy an ad blocker. Such software inevitably also presents a dilemma because they cut into the revenue of content makers.

The complex relationship between user preferences for privacy and non-intrusive ads, and the need for online entities to thrive, will continue to be a negotiation for years to come, as content makers and consumers navigate what makes for a vibrant, healthy Internet for everyone.

Further reading:

Me and My Shadow, Tactical Technology Collective
Mother in a click: pregnancy as a jackpot for the Datasucker, Chupadados, 2018
Get Smart on Tracking, Mozilla
Big data meets Big Brother as China moves to rate its citizens, WIRED, 2017