15. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: receiving placement criteria and target keywords for one 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.
15. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: receiving placement criteria and target keywords for one 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. 19. The computer storage medium of claim 15 , wherein the instructions further cause the one or more computers to perform operations comprising determining a category to which the requesting property has been assigned, the category being determined independent of the page keywords for the requesting web property.