Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, and including a method for delivering content. The method comprises identifying impressions of content to a user accessing resources using different requesting sources. The method further comprises storing impression data for the identified impressions in association with the user and requesting source. The method further comprises storing requesting source characteristic information with the impression data and identifying parameters that require limits on a number of impressions that are to occur in a time period and type of requesting source. The method further comprises receiving a request for content from a particular requesting source associated with the user, and determining when impressions available for that type of requesting source have been satisfied, and when not, enabling delivery of a content item associated with a campaign to the requesting source responsive to the received request.