> If they have no data on you, I assume ad exchanges supply default advertisements, or there are bidders on default ads? To be clear on what happens here: the

> If they have no data on you, I assume ad exchanges supply default advertisements, or there are bidders on default ads? To be clear on what happens here: the ad exchange itself isn't usually interested in identifying you per se, it's the bidders. So for example the ad exchange might send the bidders a bid request that has some information like this: Country, user agent, time zone, website that will be showing the ad, size/location of the ad, etc. Then it's up to the bidders to try to figure out how much this impression would be worth and then bid appropriately. Even if they can't 'identify' the user as someone they've seen before, they might still bid on the ad because the website showing the ad might be relevant to the product they're trying to sell. If the only information they have about a user is what is passed in the bid request they would probably bid lower. I don't know what the behaviour is for when an a bid request doesn't receive any bids, but it would be exchange-dependent, and likely rare on the major exchanges.