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Europe’s highest court this week ruled that a Spanish man can force Google(s goog) to remove a search result that contained financial information he thought was embarrassing. The surprise ruling, which has alarmed free speech advocates, has big implications for tech companies like Google and Facebook, and could fundamentally change how people use the internet in Europe.

Here’s an explanation of what the ruling actually does, what the press is saying and where to learn more.

Who is this Spanish man and what can he remove from Google?

Mario Costeja is a 59-year-old lawyer who sought to remove a Google link to a 1998 news article about an auction, which mentioned that Costeja had to sell his house to pay outstanding debts. The European Court of Justice’s decision means that the news article will remain online but that it will no longer turn up in some Google searches.

Where does the ruling apply and when will it go into effect?

The ruling applies across the European Union, but will have to be implemented by courts and law-makers on a country-by-country basis. In the case of Mr. Costeja, he will go back to a Spanish court to ask for an order consistent with the ECJ decision — one that will presumably require Google to remove the search listing.

The exact scope of the ruling is still to be determined: will Google have to purge the listing from Google.es or Google.com too? Will future court rulings apply only to searches conducted within Europe or abroad too? These questions will take years to resolve.

What does it mean for the average person?

As the Wall Street Journal notes, more than 200 people in Spain alone were already petitioning the country’s data-protection regulator to force Google to remove listings they dislike. In the wake of the ruling, many more people across Europe are likely to line up with similar demands aimed at not just Google, but at Facebook and Twitter and other sites that contain links, old photos, news items or other potentially embarrassing information.

Are there any limits on what people can take down?

Yes, the ruling suggests a request for removal must concern a link that is “inadequate, irrelevant or no longer relevant, or excessive in relation to the purpose for which they were processed and in the light of the time that has elapsed.” Courts and regulators will also have to balance the person’s desire for privacy against the “interest of the public in having that information, an interest which may vary, in particular, according to the role played by the data subject in public life.”

What does this mean for Google and other tech companies?

The ruling says Google is now considered a “data controller” and no longer a simple intermediary; it is now accountable for search listings even though it has nothing to do with the underlying content. This represents a fundamental shift in long-established principles of internet law, and presents a potentially enormous technological and regulatory headache for Google and other tech companies.

To respond to what could be millions of new legal demands, Google may attempt to create something akin an automated takedown system such as what exists for copyright and, in doing so, try to strike a balance between free expression and compliance with the law. Or Google and other companies may consider removing some or all or their services altogether.

Can Google appeal or change the law?

The ECJ is Europe’s highest court so Google can’t appeal this week’s ruling. The ruling, however, applied to 20-year-old legislation that the European Parliament is currently in the process of revising; the tech industry could lobby to have the proposed new law soften the impact of the court ruling. (That may be unlikely given other legal developments).

What has been the reaction to the ruling?

Newspapers on both sides of the Atlantic published editorials warning that the court ruling threatens the integrity of the internet and free expression. Here is an excerpt from the Financial Times:

The right to be forgotten must not become the right of the powerful to airbrush the past … the problem with the ECJ judgment is that it will create legal uncertainty and expose internet companies to disproportionate compliance costs and creeping censorship. All told, this risks undermining the worldwide web as a global commons.

And here is the New York Times:

Such a purge would leave Europeans less well informed and make it harder for journalists and dissidents to have their voices heard … The desire to allow individuals to erase data that they no longer wish to disclose is understandable. For example, there are good reasons to let people remove embarrassing photos and posts they published on social media as children or young adults. But lawmakers should not create a right so powerful that it could limit press freedoms or allow individuals to demand that lawful information in a news archive be hidden.

Google, of course, is not pleased with the ruling. According to a spokesperson: “This is a disappointing ruling for search engines and online publishers in general.”

The Computer & Communications Industry Association, an umbrella group for the tech industry, wrote: “This ruling opens the door to large scale private censorship in Europe.”

EU Justice Commissioner Viviane Reding had a very different take, however, writing in a triumphant Facebook post: “Today’s Court Judgement is a clear victory for the protection of personal data of Europeans ! … Companies can no longer hide behind their servers being based in California or anywhere else in the world.”

Where can I learn more about all this?

The legal intricacies of the ruling (PDF here) are tricky, especially for North Americans unfamiliar with the byzantine workings of the EU. The FT has so far produced the clearest and most detailed coverage (registration required), including:

Other helpful coverage includes: