1. A method for determining which advertisements to include with electronic content delivered to users over a network, comprising: storing revenue information that indicates potential revenue amounts for a plurality of advertisements, wherein each of the plurality of advertisements is associated with corresponding delivery criteria; receiving a request to provide over the network a piece of electronic content that includes a slot for an advertisement; comparing slot attributes of the slot with the delivery criteria of the plurality of advertisements to determine a subset of the plurality of advertisements that qualify for inclusion in the slot; and selecting an advertisement from the subset of advertisements to include in the slot based, at least in part, on the potential revenue amounts.