A mediation waterfall is a ranked list of advertising networks that a mobile app calls one after another, highest expected price first, until one of them agrees to fill an ad slot. The ranking rests on each network's historical or assigned effective cost per thousand impressions (eCPM), not on what any network would pay for the impression at hand. It exists because no single network fills every request in every country, and developers who compiled several networks' software development kits (SDKs) into one app needed a layer to decide whom to ask first. That layer is mediation; the waterfall is its oldest ordering rule.

How a request falls through

It starts when an app asks its mediation SDK for a banner, interstitial or rewarded video. The SDK calls the mediation server, which matches the request to a configuration by format, platform, country and ad unit. Google's AdMob calls this a mediation group. The server returns an ordered list of ad source instances, each pairing a network with a placement ID and an eCPM, and the SDK invokes each network's adapter in turn. A network that declines returns a no-fill, error code 3 in Google's Mobile Ads SDK, and the mediation layer moves on to the next adapter.

Ordering is the operator's main lever. In AdMob each waterfall source carries a manual eCPM unless optimisation is on, in which case AdMob pulls historical figures from the partner's reporting interface and re-ranks the source country by country, according to its help centre. AdMob's eCPM floors bind only its own network and bidders; third-party waterfall sources are exempt. Unity's LevelPlay offers three tier types - manual priority, Sort by CPM (cost per thousand impressions) and Optimized - plus a rate that overrides observed eCPM, set per instance, group or country. AppLovin's MAX lists "other networks" beneath its bidders, ordered by prices the publisher enters per country.

Publishers routinely place the same network several times at different floors. Prasheel Vissapragada of Google's AdMob team described the practice in a post on July 24, 2020: up to three calls to the AdMob network per request, the first at the price of the waterfall's top line item, a second at roughly half that and a third near the bottom. In a hypothetical rewarded unit, AdMob sits at $20, Network A at $15, AdMob again at $10, Network B at $8 and AdMob again at $2. If the first two decline, the impression sells for $10, even if Network B would have paid $14 for that user. It was never asked.

Hybrid setups add real-time bidding. AdMob holds an auction among bidding sources first, then slots the winning bid into the waterfall at its bid price; any waterfall source with a higher eCPM is called before it. MAX documents a similar merge: bid tokens from each SDK, bids from SDK bidders and demand-side platforms (DSPs) under the OpenRTB 2.5 protocol, then one ranking by price with the non-bidding sources.

Developers and their monetisation teams configure the stack. The mediators are Google (AdMob and Ad Manager), AppLovin (MAX), Unity (LevelPlay), Digital Turbine (FairBid) and Liftoff (Monetize). Networks such as Meta Audience Network, Unity Ads, Mintegral and InMobi supply demand. Buyers arrive through those networks or through DSPs: Display & Video 360 (DV360) has offered app mediation partners as a separate inventory source since 2024, with a fee from April that year.

From AdWhirl to unified auctions

The structure predates apps. Through the 2000s, web publishers ranked ad networks by historical eCPM and passed declined impressions down the list, an arrangement often called a daisy chain. AdWhirl, a 2009 start-up, let iPhone developers switch networks without resubmitting their apps. AdMob bought it on August 27, 2009, and its chief executive, Omar Hamoui, called the planned open-source release the "first ever open source ad mediation solution", according to Campaign. Former AdMob staff, among them Jim Payne, founded MoPub, which Twitter bought on September 9, 2013.

Publishers re-ranked networks by hand, then with tools such as AdMob's Ad Network Optimization, before bidding offered a structural fix. Facebook began testing in-app bidding on Audience Network in March 2017, and MoPub said it was experimenting with what it called advanced bidding. On March 15, 2018 Google began an AdMob beta it called open bidding. "Think of it as [Google's] Exchange Bidding, but for networks," Sissie Hsiao, its vice-president of product for mobile app advertising, told AdExchanger.

AppLovin announced on September 5, 2018 that it had bought MAX Inc., an eight-person in-app header bidding start-up chaired by Payne. Mediation was a "tough business" where innovation was slow, Adam Foroughi, AppLovin's chief executive, told AdExchanger at the time.

Consolidation followed. Facebook announced in October 2020 that Audience Network would go bidding-only, and stopped taking waterfall traffic from iOS apps on May 31, 2021 and from Android apps on September 30, 2021. AppLovin agreed on October 6, 2021 to buy MoPub for $1.05 billion in cash and shut MoPub down on March 31, 2022. Unity completed its all-stock merger with ironSource on November 7, 2022, and ironSource's LevelPlay now carries the Unity name.

Then the largest buyers stopped answering repeat calls. Google Ads said on May 1, 2023 that it would no longer respond to multiple calls in mediation from October 31, 2023, calling repeated requests for one impression "not optimal for advertiser performance". Hybrid stacks kept a reprieve until January 2024, according to Search Engine Land. DV360 followed on October 23, 2024, ending bids on repeat waterfall calls to Ad Manager and the AdMob Network.

Why it matters for marketers

The order decides who sees an impression and at what price. For advertisers, a network low in the stack reaches users only after every tier above it has declined, and a buyer answering several calls for one impression at descending floors meets the same user more than once, the practice Google cited when it withdrew. On the sell side, DataBeat reported that publishers moving from waterfalls to unified auctions cited revenue gains of 15% to 30%, figures drawn from publisher reports rather than controlled tests.

Apps kept the structure long after the web moved on, because SDK integrations are harder to replace than page tags: header bidding arrived on websites as a script, while swapping a network in an app takes a new build and an app store review.

Where it breaks

Latency is the oldest complaint. Calls run in sequence, so the worst case is the sum of every timeout in the chain. Estimates differ. Meta's business help centre says waterfall platforms make 10 to 15 calls per impression; Deconstructor of Fun, a games industry analysis site, put it at 30 to 40. Meta also claims bidding cuts ad latency by 38% to 88%, a self-reported figure.

Price blindness is structural. A historical average cannot price the impression in front of it, and in hybrid stacks an estimated eCPM still competes against a real bid.

Conflict of interest is the newest dispute. Every large mediator also sells demand - Google through the AdMob Network, AppLovin and Unity through their own ad networks - so the auctioneer is often a bidder. AppLovin's documentation describes MAX as a first-price auction giving all bidders equal access.

Mediation also cannot route around a fault inside the winning network's own SDK, as AdMob publishers found during an eight-day rewarded video failure on Android in late August 2026.

Not the same as

In-app bidding. Also called a unified auction, it offers each impression to all participating networks at once and awards it to the highest real-time bid. Most platforms run it alongside a waterfall, not instead of one.

Passback and daisy chain. The web equivalent: a declining network returns a tag redirecting the browser to the next. Mediation does the same job inside an SDK.

Demand Waterfall. An unrelated sales funnel model from SiriusDecisions, dated to 2005 or 2006, that tracks business-to-business leads.

CTV mediation. In connected television (CTV), ad servers such as Magnite's SpringServe use the word for the layer deciding which competing demand sources are called for a video break, server-side rather than in an app.

Recent developments

The waterfall now survives mostly as a fallback for networks without bidding integrations. AppLovin said in early 2025 that all 35 SDK networks on MAX were moving to bidding, some by deprecating their traditional connection. Singular reported that MAX was ending waterfall control on July 16, 2025. Josh Chandley, co-founder of Wildcard Games, told it that edits to waterfall units had been blocked and that bidding had simplified operations without yet lifting revenue. AppLovin's support pages still describe non-bidding networks ordered by price. AdMob separated its bidding and waterfall ad source tabs and added bidding eCPM floors on July 7, 2025.

Newer routes bypass the ranked list. CloudX opened app inventory to omnichannel DSPs without SDK integration on January 28, 2026. An Amazon Publisher Services bridge with InMobi added 28 bidders without publishers changing mediation settings. Prebid, the open-source header bidding project, set out a Mobile SDK 4.0 roadmap that includes new mediation adapters.

The waterfall has also become evidence. AppLovin has asked a San Francisco court to stop Unity's Ad Quality SDK from reading data on impressions AppLovin wins, including the mediation waterfall applied to each one, with participating networks and latencies. Unity denies wrongdoing and, in letters now in the court file, says MAX handles well over a majority of such auctions. The filings set a hearing on AppLovin's request for a temporary restraining order for today.

Timeline

  • 2000s: Web publishers rank ad networks by historical eCPM in sequential waterfalls, also called daisy chains
  • 2009: AdWhirl is founded in Menlo Park by Sam Yam and Ra Roath to let iPhone developers switch ad networks
  • August 27, 2009: AdMob acquires AdWhirl and promises an open-source mediation tool
  • November 9, 2009: Google announces its $750 million acquisition of AdMob
  • September 9, 2013: Twitter acquires MoPub, founded by former AdMob staff Jim Payne and Nafis Jamal
  • March 2017: Facebook begins testing in-app bidding on Audience Network
  • March 15, 2018: Google begins an open bidding beta in AdMob
  • September 5, 2018: AppLovin announces its acquisition of in-app header bidding start-up MAX Inc.
  • July 24, 2020: Google's AdMob team publishes guidance on calling its network up to three times per request
  • October 2020: Facebook announces Audience Network will become bidding-only
  • May 31, 2021: Audience Network stops accepting waterfall traffic from iOS apps
  • September 30, 2021: Audience Network becomes bidding-only on Android
  • October 6, 2021: AppLovin agrees to buy MoPub from Twitter for $1.05 billion in cash
  • January 3, 2022: AppLovin announces completion of the MoPub acquisition
  • March 31, 2022: MoPub's mediation, Advanced Bidding and marketplace shut down
  • July 13, 2022: Unity announces an all-stock merger with ironSource valued at about $4.4 billion
  • November 7, 2022: Unity completes the ironSource merger
  • May 1, 2023: Google Ads announces a move to real-time bidding for app inventory
  • October 31, 2023: Google Ads stops buying multicall waterfall requests that lack a bidding ad unit
  • January 2024: Google Ads stops responding to multicall requests in hybrid setups
  • April 2024: A DV360 fee for app mediation partner inventory takes effect
  • September 23, 2024: DV360 announces it will stop bidding on multiple waterfall calls
  • October 23, 2024: DV360 stops bidding on multiple waterfall calls to Ad Manager and the AdMob Network
  • Early 2025: AppLovin says all 35 SDK networks on MAX are moving to bidding
  • May 28, 2025: LevelPlay SDK 8.9.0 bundles Unity's Ad Quality SDK, according to Unity's changelog as quoted by AppLovin
  • July 7, 2025: AdMob separates bidding and waterfall ad sources and adds bidding eCPM floors
  • July 16, 2025: MAX ends waterfall control, according to Singular
  • October 2025: Prebid outlines its Mobile SDK 4.0 roadmap
  • January 28, 2026: CloudX opens app inventory to omnichannel DSPs without SDK integration
  • April 27, 2026: Magnite adds anomaly detection, demand path optimisation and dynamic pricing to SpringServe mediation
  • September 27, 2026: AppLovin files a JAMS arbitration demand against Unity
  • September 29, 2026: AppLovin files an application for a temporary restraining order in San Francisco Superior Court
  • September 30, 2026: Hearing on the application scheduled, according to the filings

Summary

Who. App developers and their monetisation teams build waterfalls inside mediation platforms run by Google (AdMob and Ad Manager), AppLovin (MAX), Unity (LevelPlay), Digital Turbine (FairBid) and Liftoff (Monetize). Ad networks such as Meta Audience Network, Unity Ads, Mintegral and InMobi fill them, and advertisers reach them through those networks or DSPs such as DV360.

What. An ordered list of ad network instances, ranked by manual or historical eCPM, that a mediation SDK calls in sequence until one returns an ad. Modern stacks usually run it as a hybrid, inserting the winner of a real-time auction into the list at its bid price.

When. The model came from web daisy chains of the 2000s and reached apps with AdWhirl in 2009. In-app bidding began displacing it from 2017 and 2018, Meta went bidding-only in 2021, and Google Ads and DV360 stopped answering repeat waterfall calls in October 2023 and October 2024.

Where. Inside mobile apps, between the mediation SDK compiled into the app, the mediator's server and each network's adapter, mostly in games and utility apps that earn from interstitial, rewarded and banner formats.

Why. No single network fills every request, so developers needed a rule for which to ask first. The waterfall supplied one cheaply, at the cost of latency, manual upkeep and prices set by history rather than by the impression, which is why platforms and buyers have spent nearly a decade replacing it with auctions.