Real-time bidding, almost always shortened to RTB, is a method of selling one advertising impression at a time by auction, run while the page or app that will display it is still loading. The IAB Tech Lab's OpenRTB specification, which governs most of these transactions, defines the term in a single line: bidding for individual impressions in real time, "while a consumer is waiting." The mechanism exists because inventory sold in bulk cannot be priced accurately. A thousand impressions on one website are not worth the same amount to the same buyer, and RTB moves pricing from a rate card to a computation performed at the moment of delivery.

How one impression is auctioned

The sequence begins with an ordinary ad request from a publisher's site or app. The exchange receiving it builds a bid request and broadcasts that to connected bidders. Under OpenRTB the top-level object carries an exchange-assigned idand an imp array describing the impressions offered; only those two elements are technically required. In practice it also carries a site, app or dooh object identifying the medium, plus device, user, source and regs objects supplying context, provenance and regulatory flags.

Each impression object sets its own terms. The bidfloor field states a minimum price per thousand impressions, denominated by bidfloorcur and defaulting to United States dollars. Subordinate banner, video, audio or native objects declare the format, and a pmp object attaches private marketplace deals. At the top level, at declares the pricing rule, where 1 means first price and 2 means second price plus, with exchange-specific values permitted from 500 upward. The tmax field states in milliseconds how long the exchange will wait, overriding any figure agreed beforehand.

Transport is deliberately plain. Requests travel by HTTP POST, JSON is the suggested payload format, and exchanges may offer Protobuf or Avro alternatives that compress better. The bidder answers with seat-level bid objects or declines, a no-bid being an empty response or HTTP 204. Winning bids trigger a win notice; separate billing and loss notices let a bidder distinguish an auction won from spend recorded, and learn why a bid lost. Substitution macros in those URLs return values including the clearing price.

Timings are unforgiving. Google's Authorized Buyers documentation states that the exchange waits between 80 and 1,000 milliseconds for a response, depending on format and auction type, with the exact figure in tmax. Anything later is discarded. Timeouts and unparseable responses count as errors, and Google throttles bidders producing them at a high rate. Volume is rationed separately through a callout quota in queries per second, set per bidder server URL and adjusted by recent spend.

From block lists to a monthly release cycle

RTB predates its protocol. Right Media, DoubleClick's AdX and AppNexus were auctioning impressions individually in the late 2000s, each through its own interface, and the yield manager Admeld's chief executive dated the arrival of RTB in volume to around 2010. Standardization followed practice rather than preceding it.

OpenRTB began in November 2010 as a pilot between three demand-side platforms, DataXu, MediaMath and Turn, and three sell-side platforms, Admeld, PubMatic and The Rubicon Project. Its first objective was narrow: a common format for exchanging block lists, released as version 1.0 that December. Nexage then proposed extending the work to the bid request and response themselves, producing OpenRTB Mobile 1.0 in February 2011. A video subcommittee formed around BrightRoll and Adap.tv, with DataXu and ContextWeb, and the merged document became OpenRTB 2.0 in June 2011. The IAB adopted it as a formal standard in January 2012 with version 2.1.

Later minor versions added what the market asked for: private marketplace deals in 2.2, native units in 2.3, audio in 2.4, and in 2.5 the header bidding signals, billing and loss notices still in use. Version 2.6, published in April 2022, introduced ad pods for connected television and a structured user-agent object. From release 2.6-202211 the Tech Lab incremented the version number only on breaking changes, shipping smaller updates roughly monthly.

A larger rewrite did not take. OpenRTB 3.0, developed under the Tech Lab's OpenMedia project, restructured the protocol and acknowledged that header bidding had removed the exchange's status as sole decider. Media.net announced adoption in February 2019; the Tech Lab's final 3.0 document carries a June 2020 date. Version 2.x remains the working standard. The last major consolidation ran the other way: Google retired its proprietary Authorized Buyers bidding protocol on February 15, 2025.

What the auction charges

The specification's default is second price plus, and for roughly a decade that was the market default too. Display migrated to first price between 2018 and 2019, largely because header bidding produced parallel auctions a second-price rule could not reconcile, and because buyers wanted auditable clearing prices. Under second price the winner paid the runner-up bid plus an increment, typically one cent, and could name a true valuation without penalty. Under first price the winner pays exactly what it bid, which is why bid shading became standard equipment inside demand-side platforms.

The request itself is now a cost center. PubMatic alone processes approximately 2.7 trillion bid requests a day, and queries per second ranks among the largest operating expenses an exchange carries. Publishers have adopted bid throttling; PubMatic began charging publishers $0.001 per thousand impressions on request volume above undisclosed caps from April 16, 2026; and Amazon donated its Dynamic Traffic Engine to the IAB Tech Lab on April 15, 2026 so buyers can signal in advance what they do not want sent. StackAdapt and Index Exchange cut queries per second by 71 percent by bidding on whole streaming ad breaks rather than fragmenting a 90-second pod.

Efficiency remains contested. IAB Spain's first supply-side platform guide, published April 15, 2026, found that 41 percent of programmatic spend reaches genuine measurable inventory, with direct connections delivering 70 to 80 percent of an advertiser's money to publishers against 40 to 50 percent through longer routes.

Privacy, regulation and litigation

Every bid request is a disclosure, and the regs object exists because of that, carrying gdpr, coppa, us_privacy and Global Privacy Platform strings. Regulators have not treated those fields as sufficient. The United Kingdom's Information Commissioner's Office concluded in 2019 that profiles built for RTB were "extremely detailed and are repeatedly shared among hundreds of organisations for any one bid request," and opened work with Google and IAB Europe on the protocols behind them. The Irish Council for Civil Liberties estimated in May 2022 that RTB broadcasts data about people in the United States and Europe 178 trillion times a year, a figure it called a low estimate.

The consent layer has been through court repeatedly. Belgium's data protection authority fined IAB Europe 250,000 euros in February 2022; the Court of Justice of the European Union ruled in March 2024 that consent strings are personal data; and the Belgian Market Court held on May 14, 2025 that IAB Europe is a joint controller only for those strings, not for downstream OpenRTB processing.

American litigation produced a concrete engineering change. A federal judge granted final approval on March 26, 2026 to a settlement in In re Google RTB Consumer Privacy Litigation, requiring an RTB Control that strips encrypted Google user identifiers, device advertising IDs and IP addresses from bid requests, blocks cookie matching and coarsens user-agent data. The Partner ad settings control that appeared in May 2026 implements it.

Competition law arrived in parallel. Judge Leonie Brinkema found on April 17, 2025 that Google illegally monopolized the publisher ad server and ad exchange markets, and unlawfully tied them. Closing arguments in the remedies phase concluded on November 21, 2025, with the judge signaling doubt about forcing a sale of AdX. The European Commission fined Google 2.95 billion euros in September 2025 and published a redacted decision on January 14, 2026.

Terms it is confused with

Programmatic advertising is the wider category, covering any inventory transacted by automation, including programmatic guaranteed and fixed-price arrangements that involve no auction. Around nine in ten dollars of digital display transact programmatically; not all of that clears through RTB.

OpenRTB is a specification rather than the practice. RTB names the mechanism; OpenRTB is the document most participants implement to run it, and proprietary alternatives existed alongside it until 2025.

Header bidding is a decision about where the auction happens. It moves competition into the browser or a wrapper server ahead of the ad server, and uses RTB underneath. Prebid.js 1.0 arrived in December 2017.

Private marketplaces are RTB with an invitation list. The pmp and deal objects fence an auction off for named seats, leaving request, response and timing mechanics unchanged.

Recent developments

The privacy-preserving alternative failed on adoption rather than design. OpenRTB gained igs and igi attributes in early 2024 to support Google's Protected Audience auctions; Google retired Protected Audience and most other Privacy Sandbox technologies on October 17, 2025, leaving conventional bid requests carrying the load.

Attention has shifted to what sits above the auction. Industry figures argued through January 2026 that the Ad Context Protocol is not a competitor to RTB but a portfolio layer over it, with Triton Digital chief product officer Benjamin Masse characterizing OpenRTB as the "day trading" tier. Signal quality inside the existing bidstream is being measured more closely: Amazon Publisher Services extended Signal IQ testing to the full OpenRTB specification in May 2026, covering fields including Global Placement ID and Transaction ID, the latter disputed by publishers in 2025 on yield and consent grounds. DataBeat recorded United States programmatic CPMs 51 percent above the prior year in June 2026, alongside a 12.3 percent annual decline in connected television pricing.

Timeline

  • Late 2000s: Ad exchanges including Right Media, DoubleClick AdX and AppNexus begin auctioning impressions individually through proprietary interfaces
  • November 2010: OpenRTB launches as a pilot between DataXu, MediaMath, Turn, Admeld, PubMatic and The Rubicon Project
  • December 2010: OpenRTB block list specification version 1.0 released
  • February 2011: OpenRTB Mobile 1.0 released after a proposal from Nexage
  • June 2011: OpenRTB 2.0 unifies display, mobile and video
  • January 2012: The IAB adopts OpenRTB as a formal standard with version 2.1
  • 2015: Header bidding moves auction competition ahead of the publisher ad server
  • 2018 to 2019: Display exchanges migrate from second price to first price
  • June 2019: The Information Commissioner's Office publishes its update report on real-time bidding and adtech
  • July 2019: sellers.json and the SupplyChain object are published
  • June 2020: The IAB Tech Lab's final OpenRTB 3.0 document is dated
  • February 2, 2022: Belgium's data protection authority fines IAB Europe 250,000 euros over its consent framework
  • April 2022: OpenRTB 2.6 adds ad pods for connected television and a structured user-agent object
  • May 2022: The Irish Council for Civil Liberties estimates 178 trillion RTB data broadcasts a year across the United States and Europe
  • November 2022: Release 2.6-202211 restricts version increments to breaking changes and moves to monthly updates
  • March 7, 2024: The Court of Justice of the European Union rules that consent strings are personal data
  • February 15, 2025: Google sunsets its proprietary Authorized Buyers bidding protocol in favor of OpenRTB
  • April 17, 2025: A federal court finds Google illegally monopolized the publisher ad server and ad exchange markets
  • May 14, 2025: The Belgian Market Court limits IAB Europe's joint controllership to consent strings
  • October 17, 2025: Google retires Protected Audience and most other Privacy Sandbox technologies
  • November 21, 2025: Closing arguments conclude in the United States ad tech remedies trial
  • March 26, 2026: A federal judge approves the Google RTB privacy settlement, mandating an RTB Control
  • April 15, 2026: Amazon donates its Dynamic Traffic Engine to the IAB Tech Lab
  • April 16, 2026: PubMatic's fee on excess bid request volume takes effect

Summary

Who. Exchanges and supply-side platforms conduct RTB auctions for publishers; demand-side platforms and other bidders compete in them for advertisers and agencies, identified by seat. The IAB Tech Lab maintains OpenRTB through its Programmatic Supply Chain Working Group. Google, having retired its own protocol in 2025, is both the largest exchange operator and the defendant in the litigation that has most changed the rules.

What. RTB is the sale of a single impression by auction, executed while the page or app loads. A bid request describing the opportunity is broadcast to bidders, responses are evaluated against the declared pricing rule and floor, and a winner is notified through win, billing and loss notices.

When. Per-impression auctions were running on proprietary exchange interfaces by the late 2000s. Standardization started in November 2010, reached a unified specification in June 2011 and formal IAB status in January 2012. The pricing rule changed from second price to first price between 2018 and 2019, and the last major proprietary protocol was retired in February 2025.

Where. RTB operates across web, mobile app, connected television, digital audio and digital out-of-home inventory, with OpenRTB carrying dedicated objects for each. It runs server to server over HTTP, typically in JSON or Protobuf, inside windows of 80 to 1,000 milliseconds.

Why. Selling impressions in blocks prices them all the same. RTB prices each one separately, which raises yield for sellers and precision for buyers, at the cost of broadcasting information about the audience to every invited bidder. That trade-off is the source of both the market's scale and the regulatory and legal pressure now reshaping it.