Pixel tracking is an invisible piece of code embedded on a page or in an email that records a user’s actions. It lets you measure conversions, build remarketing audiences and pass data to the algorithms that optimise campaigns. It is the foundation of measurement in online advertising: without a correctly implemented pixel, neither the ad platform nor the publisher knows what brought a result. In practice, though, you do not implement every pixel at once, because duplicated tracking leads to a situation where several sources claim the same lead. (Compare: Cookie, Tag Manager)