MUM (Multitask Unified Model) is an AI system from Google that can both understand and generate language. Google announced it on May 18, 2021. In the SEO sense, MUM is a model that powers specific features in Search and is not a ranking update by itself.
At a glance
- Type
- AI language model (Multitask Unified Model)
- Started
- May 18, 2021 (announced)
- Affects
- specific applications, not general ranking according to Google
- Completed
- ongoing (current system)
- Confirmed by
- yes
- Status
- listed as a current system in the ranking systems guide
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What can MUM do?
Google described MUM as built on the T5 text-to-text framework and as 1,000 times more powerful than BERT. It was trained across 75 languages and many tasks at once, and it is multimodal, meaning it understands text and images, with video and audio named as possible future additions.
The stated goal was to answer complex questions with fewer searches. Google cited that people issue about eight queries on average for complex tasks.
Is MUM a ranking factor?
According to Google’s ranking systems guide, MUM is active but “not currently used for general ranking in Search” and serves some specific applications. The court findings from the U.S. v. Google trial (paras. 97 to 102) differ somewhat: they list MUM among the newer ranking signals that rely less on user data, describe it as a large language model trained on a subset of the web corpus plus some click data, and state that it did not replace click-based signals such as Navboost. This entry follows Google’s documentation, last updated December 10, 2025.
- Google announces the completion of Caffeine, its new web indexing system. Google says it delivers 50 percent fresher results than the previous index.Source: googleblog.blogspot.com
- Google announces a ranking change against low-quality and copied content that noticeably affects 11.8% of queries. Google later refers to the system by its nickname “Panda”.Source: googleblog.blogspot.com
- Google improves how it decides when to favour recent results. According to Google the change affects roughly 35 percent of searches and builds on Caffeine.Source: googleblog.blogspot.com
- Google launches a change that looks at page layout. Pages with little content and many ads above the fold may rank lower.Source: developers.google.com
- Google announces a change against webspam, later nicknamed “Penguin”. It affects about 3.1% of English queries and roughly 3% of German queries.Source: developers.google.com
- Hummingbird, which Google describes as a major improvement to its overall ranking systems.Source: developers.google.com
- Panda becomes part of the core ranking systems. Google now lists it among its retired systems.Source: developers.google.com
- Google launches RankBrain, the first deep learning system used in Search. It relates the words in a query to real-world concepts.Source: blog.google
- Google starts rolling out the mobile-friendly update, boosting mobile-friendly pages in mobile search results. The industry unofficially calls it “Mobilegeddon”.Source: developers.google.com
- Penguin becomes part of the core algorithm and now works in real time. Before that, the list of affected sites was only refreshed periodically.Source: developers.google.com
- Google introduces neural matching. The system helps relate queries to pages through underlying concepts.Source: blog.google
- Launch of SpamBrain, Google’s AI-based spam prevention system. Google says it identified nearly six times more spam sites in 2021 than in 2020.Source: developers.google.com
- Google applies BERT to ranking and featured snippets. At launch it affects about one in ten English searches in the US.Source: blog.google
- Google presents passage ranking, which can assess individual passages of a page for a query. Google expects it to improve 7 percent of queries.Source: blog.google
- First product reviews update. It rewards in-depth product reviews and initially covers English-language reviews only.Source: developers.google.com
- Google introduces MUM, a model trained across 75 languages. Google calls it 1,000 times more powerful than BERT.Source: blog.google
- First link spam update. It aims to identify and nullify link spam across multiple languages and rolls out over two weeks.Source: developers.google.com
- With the helpful content update, Google aims to show more original, helpful content written for people.Source: status.search.google.com
- December 2022 helpful content update. Google says it improves the classifier and works globally in all languages.Source: status.search.google.com
- The product reviews update becomes the reviews update. Google says it now covers reviews of products, services and other things.Source: status.search.google.com
- September 2023 helpful content update. Google says it improves the classifier again.Source: status.search.google.com
- November 2023 reviews update. As the reviews system is now improved at a regular pace, Google no longer posts future updates on the Status Dashboard.Source: status.search.google.com
- The helpful content system becomes part of the core ranking systems. Google now lists it among its retired systems.Source: developers.google.com
- Three new spam policies against expired domain abuse, scaled content abuse and site reputation abuse. The March 2024 spam update launches the same day.Source: developers.google.com
- The site reputation abuse policy takes effect. Google had given site owners time to prepare since March.Source: developers.google.com
- In the US antitrust case against Google, the court describes Navboost as a signal that memorises which documents users click for a query. This is a court document, not a Google publication.Source: storage.courtlistener.com
- Google tightens the site reputation abuse policy. First-party involvement does not change the assessment when third-party content exploits the host site’s ranking signals.Source: developers.google.com
A selection, not a complete list. Core and spam updates since November 2021 have their own timeline. Names such as Florida (2003), Pigeon (2014), Possum (2016), Fred (2017) and Medic (2018), as well as Payday Loan and EMD, come from the SEO industry. Their dates rest only on trade press reports and posts by Google employees on X; Google has not published a dated page about them. Last checked: .
FAQ
Is MUM the same as BERT? No. Both are language models, but MUM is a later, larger system with multimodal input.
Sources
- Introducing MUM – Google Blog, 2021-05-18blog.google
- Ranking systems guide – Google Search Centraldevelopers.google.com
- United States v. Google LLC, Memorandum Opinion, Findings of Fact (2024-08-05)storage.courtlistener.com
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