Ad Server is specialised software to host ad content on, and deliver it to, the publishers’ websites and apps. In the course of RTB, Ad Servers make instantaneous decisions about choosing and buying impressions while the web page selected by the user is still loading. Ad Servers can also perform other tasks depending on the user needs, for example, ad metrics tracking, monitoring, and verification. On the one hand, Ad Servers are used for ad hosting and serving, which undoubtedly adds great value to them from the publishers’ perspective. On the other hand, Ad Servers are used by advertisers to track ad impressions and assess advertising campaign efficiency.