What Google says
In Google's own words: “Placement exclusions allow you to exclude your ads from serving on placements for specific pages, sites, mobile apps, and videos for brand suitability purposes.” (Google help center)
What you can exclude, and where
You can exclude individual URLs, whole sites, mobile apps, entire app categories (games, dating, kids), YouTube channels, and videos. For most campaign types you set these at the campaign level. For Performance Max, campaign-level exclusions are not available, so they must be applied at the account or manager-account (MCC) level, which PMax then respects.
The two-layer habit
Category exclusions through content suitability knock out the obviously off-ICP inventory up front. The ongoing work is the placement report ("Where ads showed"): review it regularly and exclude the specific apps and sites that actually served junk. Placements never stop appearing, so it is recurring hygiene, not a one-time setting. Placement exclusions are the where-ads-show cousin of negative keywords; full walkthrough in the PMax mobile-app leak. They sit alongside the account-level content suitability setting, which controls how sensitive the surrounding content can be; we compare the two in Google Ads inventory types explained.
A worked example
Say a Display campaign spends $3,000 a month and the placement report shows $900 of it (30%) went to 40 mobile game apps with zero conversions. Excluding those apps at the account level returns that $900 to placements that convert; at a $45 cost per acquisition that is room for 20 more conversions a month. The same account-level list also covers Performance Max, which cannot take campaign-level placement exclusions.
Common questions
What is a placement exclusion?
A placement exclusion (or negative placement) prevents your ads from showing on a specific website, app, app category, YouTube channel, or video. It is how you keep Display and Performance Max campaigns off irrelevant or low-quality inventory.
Can you exclude placements in Performance Max?
Yes, but only at the account or manager-account (MCC) level. Performance Max does not support campaign-level placement exclusions, so you apply them account-wide and PMax respects them. You can also exclude whole app categories through content suitability settings.
How do I exclude placements in Performance Max?
Add them at the account level: in Content suitability under Advanced settings, open Excluded placements and add the sites, apps, YouTube channels, or videos. Performance Max respects all account-level and manager-account (MCC) placement exclusions, and account-level exclusions override campaign and ad group settings.
How do I build a YouTube placement exclusion list?
Placement exclusion lists are created in a manager account and applied to linked child accounts, where they affect all relevant campaigns including Performance Max. In a single account, you can paste YouTube channel IDs or video URLs into the account-level Excluded placements setting. You cannot exclude all of youtube.com as a placement.
How many placements can I exclude?
Up to 65,000 placement exclusions per account, entered at most 20,000 at a time. Manager accounts can create up to 3 placement exclusion lists with a maximum of 250,000 exclusions per list.