What is an ad server?
An ad server is software that decides which ad to show in a given slot, delivers it and counts impressions and clicks. Publishers use a publisher ad server to manage their inventory, direct deals and programmatic demand; advertisers use an advertiser ad server to host creatives, track delivery across sites and run independent measurement.
A publisher ad server such as Google Ad Manager decides between direct-sold campaigns, programmatic guaranteed deals and auction bids according to priority and price. An advertiser ad server such as Google Campaign Manager 360, Flashtalking or Innovid serves the creative and provides a neutral count for billing and attribution.
Because the publisher ad server sees every opportunity, it is powerful. The US court's 2025 ruling found Google had monopoly power in publisher ad servers (DFP) and unlawfully tied it to its AdX exchange; the 2026 remedies ruling ordered behavioral changes rather than a sale.