1. A computer-implemented method for delivering content to a user, comprising: identifying previous impressions of content to a user, each previous impression including a time that the particular impression was previously presented to the user, wherein identifying impressions includes identifying impressions to the user accessing resources using different requesting sources; storing impression data for the identified impressions in association with the user including storing impression data on a per requesting source basis such that impressions that were received on a respective requesting source can be identified; storing requesting source characteristic information that characterizes a given requesting source in association with the impression data; identifying information including parameters that require limits on a number of impressions that are to occur in a time period for a particular type of requesting source wherein the information includes sequencing information for sequencing content items in the campaign, and wherein identifying impressions and storing impression data further includes identifying a sequence of impressions of content items from the campaign; receiving a request for content to be displayed on a particular requesting source associated with the user; and determining, using one or more processors, when impressions available for that type of requesting source have been satisfied based at least in part on the impression data and the parameters, and when not, enabling delivery of a content item associated with a campaign to the requesting source responsive to the received request.