The Google ad tech case, explained

For more than a decade, Google has owned the most widely used tools on both sides of display advertising: the publisher ad server most big websites use, the biggest ad exchange, and major buying tools for advertisers. In 2025 a US federal court ruled that Google had broken antitrust law in how it ran that stack, and in 2026 the same court decided what Google must change. This is the most important competition case the open web has seen.

Imagine one company owns the auction house, represents most of the sellers, and also sends in many of the bidders. Even if each part works well, it can quietly give its own bidders a peek at rival bids or make sellers use its auction house to reach its bidders. That is the heart of the complaint.

The three pieces of Google’s stack

Timeline

1. January 2023: the lawsuitThe US Department of Justice, joined by a group of states, sued Google in the Eastern District of Virginia, alleging it monopolized ad tech markets. This is United States v. Google.
2. September 2024: the liability trialA three-week bench trial before Judge Leonie Brinkema, with no jury.
3. 17 April 2025: liability rulingThe court found Google had unlawfully monopolized the markets for publisher ad servers and for ad exchanges for open-web display advertising, and had unlawfully tied DFP and AdX together. It rejected the government’s claim that Google monopolized a market for advertiser ad networks.
4. September 2025: remedies trialThe government asked the court to force Google to sell AdX and to open up DFP’s auction logic, potentially followed by a sale of DFP. Google proposed behavioural changes instead.
5. September 2026: remedies decisionJudge Brinkema rejected a forced sale of AdX and ordered a package of behavioural remedies, described below. The full opinion was unsealed on 16 September 2026.

What the remedies ruling ordered

According to reporting on the unsealed opinion by AdExchanger, the court chose behavioural rules rather than a break-up. The main points reported were:

• No divestiture of AdX: the structural remedy the government wanted was rejected.
• Prebid integration: Google must build ways for AdX and DFP to work with Prebid, the open-source header bidding system.
• Equal access for rival ad servers: AdX must send real-time bids to competing publisher ad servers on the same terms as to DFP.
• Ending self-preference: “first look” and “last look” advantages are prohibited, Unified Pricing Rules are to be deprecated so publishers can set different floors per bidder, and Google Ads may not bid directly into DFP in a way that favours Google’s own tools.
• A technical monitor for six years, with access to Google’s systems, shorter than the 15 years the government requested.
• Not ordered: open-sourcing DFP’s auction logic, and restrictions on DV360.

The European case

On 5 September 2025 the European Commission fined Google €2.95 billion for abusing its dominance in ad tech. The Commission found Google favoured its own exchange AdX both through its publisher ad server DFP, for example by telling AdX in advance about the value of competing bids, and through its buying tools Google Ads and DV360. It ordered Google to stop these practices and to propose, within 60 days, how it would end the conflicts of interest across the chain. The Commission said that only divestiture of part of the business might fully resolve them, but left the first proposal to Google. Google said it would appeal. Read the summary from the Law Society Gazette.

Why it matters beyond Google

Other competition authorities, including Canada’s Competition Bureau and the UK’s Competition and Markets Authority, have also challenged Google’s ad tech conduct. The practical effects for the industry are likely to be more open auctions, more room for independent SSPs and ad servers, and more pressure on any company that owns both sides of a marketplace. The case also turned technical details that most people never see, such as auction timing and floor rules, into questions of law. That is a lesson for all of antitrust in ad tech in ad tech: small design choices in an auction can decide who wins a market.