Systems and associated methods for targeted ad delivery are described. In one aspect, a user profile includes at least one user content category identifier and a user interest score. The user interest score is increased by an amount if the content category identifier of a requested webpage matches the at least one user content category identifier. In a further aspect, the user interest score is indicative of a level of interest of the user in a content category identified by the at least one user content category identifier. In another aspect, the user interest score can be reduced according to a long-term decay rate that slowly reduces the user interest score. In another aspect, the user interest score can also be reduced according to a short-term decay rate that rapidly reduces the user interest score.