8. A system comprising: a data store storing targeting criteria for a set of advertisements for one or more campaigns, the targeting criteria for each advertisement in the set including: placement criteria with which presentation of the advertisement is conditioned independent of page keywords for a web property; and target keywords with which presentation of the advertisement is conditioned based on a match existing between one of the target keywords and the page keywords for the web property; and one or more computers configured to interact with the data store, the one or more computers being further configured to perform operations comprising: receiving the placement criteria and the target keywords for theone or more campaigns; receiving a content item request that includes data specifying a requesting web property for which a content item is being requested; identifying, based on the received placement criteria, advertisements that are targeted using placement criteria that identify at least one page of the requesting web property, the identification of the advertisements based on the placement criteria being independent of page keywords of the requesting web property; selecting, as an advertisement that is eligible to be presented with the requesting web property, one of the identified advertisements that, in addition to being targeted using the placement criteria, is further targeted using target keywords that are matched by page keywords for the requesting web property; and providing the selected advertisement in response to the content item request.
8. A system comprising: a data store storing targeting criteria for a set of advertisements for one or more campaigns, the targeting criteria for each advertisement in the set including: placement criteria with which presentation of the advertisement is conditioned independent of page keywords for a web property; and target keywords with which presentation of the advertisement is conditioned based on a match existing between one of the target keywords and the page keywords for the web property; and one or more computers configured to interact with the data store, the one or more computers being further configured to perform operations comprising: receiving the placement criteria and the target keywords for theone or more campaigns; receiving a content item request that includes data specifying a requesting web property for which a content item is being requested; identifying, based on the received placement criteria, advertisements that are targeted using placement criteria that identify at least one page of the requesting web property, the identification of the advertisements based on the placement criteria being independent of page keywords of the requesting web property; selecting, as an advertisement that is eligible to be presented with the requesting web property, one of the identified advertisements that, in addition to being targeted using the placement criteria, is further targeted using target keywords that are matched by page keywords for the requesting web property; and providing the selected advertisement in response to the content item request. 14. The system of claim 8 , wherein the one or more computers are further configured to perform operations comprising: identifying page keywords for the requesting web property; identifying target keywords for each of the identified advertisements; determining that at least one of the target keywords matches the page keywords; and selecting, as the advertisement that is eligible to be presented with the requesting web property, one of the identified advertisements that is targeted using the at least one target keyword.