DoubleClick uses cookies to display adverts that are relevant to the user, to improve reports on campaign performance, and to avoid users being shown the same adverts multiple times. Via a cookie ID, Google records which adverts are displayed in which browser and is thus able to prevent the adverts from being shown multiple times. Furthermore, with the aid of cookie IDs, DoubleClick can record conversions relating to ad requests. This is the case, for example, when a user sees a DoubleClick advert and later visits the advertiser’s website using the same browser and makes a purchase on that website.