Video Ad Serving Template (VAST)

The Video Ad Serving Template (VAST) is a standardized communication protocol developed by the Interactive Advertising Bureau (IAB) that facilitates the delivery of video advertisements between ad servers and video players. It defines a universally accepted XML-based schema that enables consistent formatting, tracking, and playback of video ads across different platforms and devices—including Connected TV (CTV), desktop, mobile, OTT apps, and game consoles.

First introduced in 2008 and continuously updated to support the evolving demands of digital video advertising, VAST has become the de facto standard for serving linear and non-linear video ads in digital environments. Its adoption ensures interoperability across a fragmented ecosystem, making it foundational to the programmatic and direct delivery of ads in ad-supported video content.

What does VAST do?

VAST serves as the translator between the video player (which renders content to the viewer) and the ad server (which supplies the advertising creative and tracking assets). It does this by delivering a structured XML response that includes:

• URLs for the ad creative (the video file or interactive unit)
• Tracking pixels for impressions, viewability, completions, and other ad events
• Instructions for ad type, duration, placement (pre-roll, mid-roll, post-roll), and skippability
• Support for companion ads, interactive overlays, and fallback creatives
• Metadata to support ad verification, measurement, and reporting

In short, VAST enables the standardized communication needed for video ads to be reliably delivered, rendered, and measured across a wide range of platforms.

Why is VAST critical in Connected TV (CTV)?

As CTV becomes a dominant channel for video consumption, VAST plays a crucial role in enabling ad monetization at scale across a wide array of CTV platforms, devices, and operating systems. Unlike traditional TV, which relies on fixed ad delivery methods, CTV supports dynamic, addressable advertising that requires real-time communication between ad servers and video players.

VAST ensures that regardless of the CTV device—whether it’s a Roku stick, Samsung smart TV, LG WebOS, Android TV box, or a PlayStation app—the same ad logic and structure can be used to serve, play, and track video ads consistently.

This consistency is essential in a fragmented CTV landscape where multiple streaming services, devices, and video players each have their own specifications. VAST provides the common language that allows advertisers to scale campaigns and video publishers to ensure monetizable playback across all viewing environments.

Key components of a VAST response

A typical VAST XML response includes several core elements that dictate how the video ad is to be delivered and tracked:

• <Ad>: The root element that defines an individual ad unit
• <InLine> or <Wrapper>: Specifies whether the ad content is delivered directly or via a redirect to another ad server
• <MediaFile>: Provides the actual URL of the ad creative to be rendered
• <TrackingEvents>: Contains tracking pixels for impression, start, quartile completions, mute, pause, resume, and other player events
• <ClickThrough>: The destination URL when a user clicks the ad
• <CompanionAds>: Optional banners or static images that accompany the video ad
• <Extensions>: Custom data, such as ad verification tags, interactive elements, or custom macros

VAST and ad playback types

VAST supports a wide variety of ad formats and playback models, including:

• Linear ads: Standard pre-roll, mid-roll, or post-roll video ads that play sequentially with content
• Non-linear ads: Overlay banners or pop-ups that appear simultaneously with video content
• Skippable and non-skippable ads: Ads that may or may not allow user skipping after a set duration
• Interactive video ads: Creative formats that support user engagement, such as choosing a path or clicking for more information
• VAST wrappers: Used when multiple ad servers are chained together to facilitate waterfalling or supply path optimization

This flexibility allows VAST to support a range of monetization strategies, from direct-sold campaigns to programmatic auctions.

VAST and measurement

VAST is deeply integrated into the ad measurement and verification ecosystem. It enables advertisers to track:

• Ad impressions
• Viewability
• Quartile completions (25%, 50%, 75%, 100%)
• Click-throughs
• Mute/unmute events
• Ad errors and playback issues

This tracking is essential for performance analytics, billing, campaign optimization, and ensuring advertiser ROI. VAST also supports third-party verification tools via ad extensions, enabling integrations with vendors like MOAT, IAS, and DoubleVerify.

Challenges and evolution of VAST

Although VAST has streamlined video ad delivery, earlier versions had limitations around creative compatibility, interactivity, and cross-device reliability. These gaps led to the development of complementary standards such as:

• VPAID (Video Player Ad-Serving Interface Definition): For interactive ad experiences (now deprecated in favor of more secure alternatives)
• SIMID (Secure Interactive Media Interface Definition): A modern replacement for VPAID that separates interactivity from media files
• OMID (Open Measurement Interface Definition): Standard for ad viewability and verification

VAST has evolved to incorporate more robust capabilities, such as VAST 4.0+, which supports mezzanine files (for creative transcoding), standardized error reporting, and better support for server-side ad insertion (SSAI).

VAST and SSAI

In Server-Side Ad Insertion (SSAI) workflows, VAST plays a slightly different but equally critical role. The SSAI system calls the VAST endpoint to retrieve ad creatives and metadata, which are then stitched into the video stream at the server level. This enables seamless playback on CTV devices and better ad-blocking resistance. Even in SSAI environments, VAST is used to track impressions, events, and errors, providing the foundation for performance analysis and campaign reporting.

Final thoughts

The Video Ad Serving Template (VAST) is a cornerstone of modern digital advertising, especially in the era of Connected TV. It enables standardized, scalable ad delivery and tracking across devices, supports multiple ad formats and monetization models, and provides the foundation for robust measurement and optimization.

As CTV continues to grow, and advertisers demand more precision, consistency, and control, VAST will remain at the heart of how ads are delivered and monetized in premium video environments.