Online video dominates media today – streaming recently accounted for nearly 50% of total TV viewing time in mid-2025. Advertisers naturally follow viewers, inserting pre-roll, mid-roll, and other ads into video content across devices.
To the viewer, this is a simple idea, and the mechanism behind the execution is never really seen—they just know there’s a short video ad playing before their intended video. But behind the scenes, a complex system serves as a communicator between the video platform of choice and the ad server. There are several different types of systems, but one of the most efficient and most popular is VAST.
What is this system? How is VAST history evolving? How can you use it to improve your advertising campaign?
Table of Contents
What Is VAST?
VAST is an acronym for Video Ad Serving Template, and it’s an XML structure that helps the video player decide what ad to play next. Basically, it determines three things:
- The video content for the ad that needs to be played.
- Instructions for how the ad should be played.
- Any tracking metrics necessary for the ad’s display.
Sounds pretty simple, right? But the XML structure behind VAST is an impressively complex system, and it keeps getting better.
VAST History in All Versions
VAST was first introduced by IAB, the Interactive Advertising Bureau, as version 1.0 back in the mid-2000s. As video players became more sophisticated and the demands of video advertisers became more complex, the system underwent a series of different iterations leading up to VAST 3.0 back in 2012. For several years, 3.0 was treated as the pinnacle of VAST development, but in November 2015, an even newer version was released—4.0. VAST 4.0 (2016) brought server-side ad stitching, mezzanine files, multiple renditions, and universal IDs. Subsequent updates—4.1 (2017) with audio and SSAI support, 4.2 (2019) with SIMID for secure interactivity, and 4.3 (2022) with CTV-focused tools like idle-viewer detection—kept pace with new formats and devices. Recent addenda (2023–2024) further enhanced privacy-safe attribution, CTV creatives, and universal creative IDs, making VAST 4.x the backbone of digital video advertising in 2025, with a possible JSON-based VAST 5.0 still on the horizon.
Key Enhancements in Modern VAST
Recent VAST versions and addenda include many powerful features for publishers and advertisers.
- Server-Side Ad Insertion (SSAI): VAST fully supports server-side stitching of ads into streams. When a server stitches ads into the content, it still uses VAST tags for tracking, but the delivery is seamless to the player. The current VAST spec explicitly accommodates SSAI workflows. This means ads can be pre-stitched into video (or audio) streams, avoiding buffering glitches and bypassing client-side ad blockers. Publishers report much smoother playback on CTV/OTT devices when using SSAI in compliance with VAST.
- Interactive and Rich Ad Formats: The integration of SIMID into VAST 4.2 marked a new era for interactive ads. SIMID allows JavaScript-based interactivity (animations, polls, overlays, etc.) to run in a secure, player-controlled sandbox. In practice, a VAST 4.2+ tag can reference a SIMID creative instead of a VPAID SWF/JS file. This preserves all of VPAID’s creative flexibility but resolves its security issues. Concurrently, VAST now works hand-in-hand with the Open Measurement (OMID) SDK, so viewability and fraud verification code can easily attach to any VAST ad.
- High-Quality Media Support: VAST 4.0 introduced the concept of raw “mezzanine” media files and adaptive streams. Publishers can now supply ads at multiple resolutions or bitrates within a single VAST tag. A modern player will select the highest appropriate stream (e.g. 1080p or 4K when bandwidth allows). The specification also separated creative metadata from media URLs, so even large uncompressed source files can be referenced and transcoded on demand. These capabilities ensure that video ads play at optimal quality on any device or screen, from mobile phones to large TVs.
- Connected TV Enhancements: With many ads now viewed on smart TVs and streaming boxes, VAST has gained CTV-specific tools. VAST 4.3 can query “still there?” prompts built into many TV OSes – if the viewer does not confirm, the ad can pause or switch to a filler, preventing billing for unviewed inventory. Additionally, the protocol allows publishers to request the actual ad asset (video file) before play, so they can validate its content and format on-the-fly. A 2024 CTV addendum added new icon elements (for on-screen overlays like promos or skip buttons) and explicitly optimized tags for large-screen delivery. Taken together, these features make VAST-driven campaigns much more robust on TV apps and devices.
- Universal Creative IDs (ACIF): To streamline cross-platform campaigns, IAB introduced the Ad Creative ID Framework. Each creative can carry a single unique ID that persists across publishers and devices. VAST tags in 2024+ can include this ID to ensure consistency in frequency capping and reporting, regardless of where the ad appears. In fact, IAB has published an addendum requiring standardized creative ID fields in VAST tags (even as far back as VAST 2.0). This means media buyers and sellers can now reconcile a campaign by ID across disparate log systems, reducing errors and double-counting.
- Privacy-Friendly Attribution: With third-party cookies waning, video ad attribution is evolving. A recent IAB addendum lets publishers mark impressions and clicks in VAST for Google’s new Attribution Reporting API. When the video player fires that impression URL, the browser can register it for later conversion matching. Importantly, this is designed as an opt-in mechanism: VAST documents can include special flags or attributes that tell the platform to register those events in a privacy-safe way. This feature works with all versions of VAST, ensuring older tags still benefit from cookie-less attribution support.
Each of these enhancements is backward-compatible: players and servers that support newer versions will ignore unknown fields in older tags, and vice versa. In practice, this means a modern ad server can output a single VAST 4.3 tag that drops back to older behavior on legacy players, while taking advantage of all the new bells and whistles on updated platforms.
Where to Learn More
In this article, we’ve covered the basic history of VAST and why new versions of VAST history offer such a significant improvement. However, we’ve only scratched the surface of what VAST can do—let alone how to implement it for your campaign. For more information, be sure to check out IAB’s official documentation on the new system.
In addition, IAB Tech Lab press releases and blogs explain new capabilities in plain language. Advertisers and publishers should reference these materials when implementing VAST to ensure compatibility. Industry webinars and ad tech documentation also walk through practical integration steps.
A New Era of Video Ads
Today’s video advertiser needs to strike a balance between two key areas of development: creative messaging and powerful technology to support it. The VAST 4.x standard provides a robust, streamlined framework on the tech side – from multi-bitrate streaming to viewability measurement to privacy-safe reporting. With VAST’s latest features and the resources above, advertisers and publishers have a robust toolkit for launching effective video ad campaigns in 2025 and beyond.