Abstract: A system and methods are provided for serving content in response to content queries or requests. When a request is received, for content to be presented to a specified user, candidate content items are identified, possibly based on matches between attributes of the specified user and attributes of the items' target audiences. For each item, a history indicating the frequency (e.g., total number) and/or recency with which impressions of the candidate item (and/or related items) were previously presented to the specified user is retrieved and used to filter out candidate content items and/or collections of content items that have been already been served to the specified user a threshold number of times. Each remaining item's estimated value is computed, the results are ranked and the top-ranked item(s) are served.