VAST (Video Ad Serving Template) is a standardized protocol developed by the IAB (Interactive Advertising Bureau) that defines how video ads are served to and displayed within video players. It's an XML-based template that acts as a universal language between ad servers and video players: when a video ad is requested, the ad server returns a VAST response — an XML document — that tells the player everything it needs to know to load and display the ad correctly. VAST is the technical foundation that makes programmatic video advertising work across the fragmented landscape of players, platforms, and devices.
 

The problem VAST solves is interoperability. Without a standard, every video player and every ad server would need custom integrations to communicate, making scaled video advertising impossible. VAST standardizes the conversation. The XML response specifies the location of the video creative file, the ad's duration, which type of ad it is (pre-roll, mid-roll, post-roll), what tracking events to fire (start, quartiles, completion, clicks), the clickthrough destination, and any companion ads or additional creative elements. Any VAST-compliant player can parse this response and serve the ad correctly, regardless of which ad server produced it.
 

VAST tracking is particularly important. The template defines tracking URLs that fire at key moments in the video's playback — when the ad starts, reaches 25%, 50%, and 75% completion, finishes, is clicked, or is skipped. These events feed the measurement that advertisers depend on: view-through rates, completion rates, engagement, and viewability. This standardized tracking is what lets advertisers measure video campaign performance consistently across diverse inventory.
 

VAST has evolved through multiple versions, each adding capabilities — support for skippable ads, better error handling, ad verification, and more. It's often discussed alongside two related standards. VPAID (Video Player-Ad Interface Definition) added interactivity and richer measurement to video ads but introduced complexity, security, and performance concerns, and has largely been deprecated in favor of newer approaches. VMAP (Video Multiple Ad Playlist) handles the scheduling of multiple ad breaks within a single video, working in concert with VAST to define where and when ad breaks occur. The Open Measurement SDK (OM SDK) has taken over much of the viewability and verification role VPAID once served.
 

For the programmatic video ecosystem, VAST is indispensable. It enables in-stream and out-stream video ads to be bought and sold programmatically and served seamlessly across the enormous diversity of video environments — desktop, mobile web, in-app, and connected TV. Whenever a video ad plays before your content or interrupts your stream, VAST is almost certainly the standard orchestrating it behind the scenes. For anyone working in video advertising, understanding VAST is fundamental to understanding how video ads are delivered, tracked, and measured at scale.