VAST (Video Ad Serving Template) is an XML-based standard defined by the IAB Tech Lab that specifies how a video ad gets delivered from an ad server to a video player. When a video player needs to show an ad, it sends an ad request to an ad server. The server returns a VAST response — an XML document — that contains the video file URL, click-tracking URL, viewability beacons, and other metadata. The player parses VAST and plays the ad.
Why it matters
- Universal compatibility — virtually every video player and every ad server speaks VAST. Without it, every ad-server / video-player pair would need a custom integration.
- Tracking + viewability — VAST defines exactly how impression beacons, quartile tracking, click-tracking, and viewability events fire. Standardized so advertisers can verify delivery across vendors.
- Wrapper support — VAST responses can include "wrapper" elements pointing to another VAST URL (so multiple ad networks can chain through to a final creative). Useful in mediation and waterfall setups but adds latency.
VAST versions: - VAST 2.0 (2009): the foundational version. Still supported for compatibility. - VAST 3.0 (2012): adds skippable / non-skippable signaling, additional tracking events. - VAST 4.0 / 4.1 / 4.2 (2016-2020): separates the creative file from companion / icon elements, adds support for ad verification (better OM-compliant viewability), adds mezzanine file support for higher-quality CTV ads. Most modern integrations use VAST 4.2 with VAST 3 / 2 fallback for older players.
Related standards: VPAID (Video Player Ad Interface Definition) is a JS-based extension to VAST that lets ads run interactive code inside the player. Largely deprecated for new mobile ads in favor of OMID / SIMID. MRAID (covered separately) is the mobile-specific rich-media counterpart.