A bid request is the message sent within the real-time bidding (RTB) system that announces an available ad impression and invites buyers to bid on it. When a user loads a page or opens an app, the publisher's supply-side platform (SSP) — or an ad exchange or ad network — packages up everything known about that impression and broadcasts it to connected demand-side platforms (DSPs). The bid request is the opening move of every programmatic auction.
 

The richness of a bid request determines how intelligently buyers can bid. A typical request, formatted according to the IAB's OpenRTB protocol, includes details about the impression and its context: the publisher and page or app, the ad slot's size and format, the device type and operating system, the user's geographic location, connection type, and available audience signals or identifiers, plus any applicable floor price. The more accurate and complete this information, the better DSPs can evaluate whether the impression matches an advertiser's targeting and how much it's worth.
 

On the DSP side, the bid request triggers a rapid decisioning process. The platform checks the impression against every active campaign's targeting criteria, audience segments, frequency caps, and brand-safety rules. For impressions that qualify, it calculates an optimal bid based on the predicted value of the impression to the advertiser, then returns a bid response. All of this must happen in milliseconds, because the auction resolves and the ad serves before the page finishes loading.
 

For advertisers, understanding bid requests is key to supply-path optimization and bidding efficiency. Requests that arrive through transparent, high-quality supply paths with rich data are worth more and easier to value accurately. Requests stripped of useful signals, or arriving through opaque or duplicative paths, carry more risk of waste and fraud. Smart buyers filter and prioritize the request streams they bid on.
 

For publishers, the bid request is how inventory is exposed to competitive demand. Sending well-formed, data-rich requests to many quality buyers increases competition and lifts the winning price. Header bidding multiplies this effect by generating simultaneous requests to multiple SSPs and exchanges, ensuring every eligible buyer can compete for each impression.
 

Privacy regulation and the decline of third-party identifiers are reshaping what bid requests can contain, pushing the industry toward contextual signals, privacy-safe identity, and first-party data. But the bid request remains the foundational event of programmatic: the moment an impression goes to market.