1. A computer-implemented system comprising: one or more rule repositories comprising at least one rule; one or more content items, wherein at least one of the content items is associated with at least one of the rules; one or more servers operable to: receive a content item request for content items to be presented to a viewer from a requesting system; receive input, wherein the input comprises information about the number of times a content item has been previously presented; evaluate one or more of the rules to determine a set of content items, wherein at least one of the content items in the set is associated with a priority, wherein the priority is based, at least in part, on the input; and prioritize at least a portion of the content items in the set.