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A supply-side platform (SSP) helps publishers sell advertising inventory; a demand-side platform (DSP) helps advertisers and agencies evaluate and buy ad impressions. In a common programmatic route, they connect through an auction, but not every online ad uses both platforms.
What is a supply-side platform (SSP)?
An SSP is software used by publishers, apps, and other media owners to manage and offer advertising inventory to potential buyers. Its central job is to help the seller monetize available ad space by connecting that inventory with demand sources. IAB Europe’s supply-chain Q&A and Amazon Ads’ SSP guide describe this publisher-side role.
What is a demand-side platform (DSP)?
A DSP is software used by advertisers and agencies to select and buy ad impressions from multiple sources. It helps assess an opportunity against campaign goals and settings, including the audience, placement, bid, and budget. The DSP represents the buyer’s side of the transaction, rather than the publisher selling the space. IAB Europe and Amazon Ads outline these roles.
SSP vs. DSP: the practical difference
| Aspect | SSP | DSP |
|---|---|---|
| Main customer | Publisher, app, or media owner | Advertiser or agency |
| Core job | Manage and sell advertising inventory | Select and buy ad impressions |
| Optimization focus | Inventory monetization and yield | Audience, placement, bid, budget, and campaign outcomes |
| Direction in the transaction | Connects inventory to demand sources | Connects buying decisions to publishers, SSPs, and exchanges |
These are role distinctions, not a guarantee that every ad-tech company performs only one function. Some companies operate at multiple levels of the supply chain, as the UK government’s Online Advertising Programme consultation notes.
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How an SSP and DSP can work together
A useful simplified example is an open-display programmatic transaction:
- A person loads a page or app, creating an available ad impression.
- The publisher’s ad server signals the opportunity through its SSP.
- The SSP makes the opportunity available to demand sources.
- DSPs assess it against advertisers’ campaign goals and settings; a DSP may submit a bid on an advertiser’s behalf.
- The applicable auction process determines an outcome, and the selected ad is delivered.
This is a teaching model, not a universal sequence. Auction formats and execution flows vary. IAB Tech Lab defines a programmatic auction as “part of the process to determine how an advertisement is delivered into a relevant ad placement”; its definitions page was finalized June 24, 2026. IAB Tech Lab’s programmatic-auction definitions describes the terminology and variation.
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Does every online ad use both an SSP and a DSP?
No. The SSP-to-auction-to-DSP diagram describes one common programmatic route, not a required path for every online ad. Buying routes can range from a fuller programmatic supply chain to direct contracts between an advertiser and publisher. Some publishers sell inventory without SSP or DSP involvement, and some ad-tech businesses span multiple supply-chain roles, according to the UK government consultation. Direct deals and overlapping roles mean the chain should not be treated as a fixed set of separate companies.
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How buyers can check the supply path
Tools including ads.txt, sellers.json, and the SupplyChain object can help buyers check who is authorized to sell inventory, understand the publisher supply chain, and identify counterfeit inventory before bidding. IAB Europe’s 2025 Q&A reports findings from a specific sample of European online news publishers:
- At least 73% of 2,054 publishers in the sample hosted ads.txt files.
- More than 99% of ads.txt lines checked in the sample were valid.
- Around 79% of the sellers.json relationships checked matched publisher declarations.
These are sample-specific results reported by IAB Europe in 2025, not global adoption rates or a measure of every publisher’s current setup.
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