1. A computer-implemented method for determining which advertisements to include with electronic content delivered to users over a network, the method comprising the steps of: after accepting a first contract with a first advertiser, accepting a second contract with a second advertiser; wherein the delivery obligations associated with the second contract are such that fulfillment of the second contract would prevent the delivery obligations of the first contract from being fulfilled; receiving a plurality of advertisements from a plurality of advertisers; storing revenue information that indicates potential revenue amounts for the plurality of advertisements, wherein each of the plurality of advertisements is associated with corresponding delivery criteria and a corresponding contract of a plurality of contracts; wherein the plurality of contracts includes the first contract and the second contract; wherein the plurality of advertisers includes the first advertiser and the second advertiser; receiving, from a client that is not one of the plurality of advertisers, a request to provide over the network a piece of electronic content that includes a slot for an advertisement; and in response to receiving the request, performing the steps of: one or more computing devices comparing slot attributes of the slot with the delivery criteria of the plurality of advertisements to determine a first subset of the plurality of advertisements that qualify for inclusion in the slot, wherein the slot attributes of the slot include at least one of (a) the nature of the piece of electronic content, (b) the size of the slot within the piece of electronic content, or (c) the placement of the slot within the piece of electronic content; the one or more computing devices creating a second subset of advertisements by filtering, out of the first subset, based on behindness values computed for the advertisements, advertisements whose delivery obligations are on track to be satisfied; wherein the second subset includes a first advertisement associated with the first contract and a second advertisement associated with the second contract; wherein the second contract is associated with a behindness value that is currently greater than a behindness value associated with the first contract; wherein the behindness value of each contract reflects how far behind a content provider is on satisfying the delivery obligations associated with each contract; and selecting the first advertisement from the second subset of advertisements to include in the slot based, at least in part, on the potential revenue amounts; inserting said first advertisement into the slot to create a modified piece of electronic content; delivering, as a response to the request, the modified piece of electronic content to the user.