1. A computer-implemented method performed by one or more processing devices, the method comprising: receiving structured data describing a content item, the structured data including one or more data elements comprising one or more descriptions and values for the one or more data elements, the structured data indicating a category for the content item; retrieving stored data for the category, the stored data identifying one or more domains; forming a first query based on data elements in the structured data; searching one or more of the domains, using the first query, to identify resources associated with the one or more domains; determining one or more queries based on data reflecting past search queries, where each of the one or more queries resulted in one or more of the resources being returned as part of a search result; determining keywords based on the one or more queries; transmitting or storing the keywords for use in impression allocation decisions; determining an allocation score for the content item based at least in part on comparison of the keywords to a term associated with a request for an impression, where the allocation score is used to determine whether the content item will be selected for the impression; selecting the content item for display in response to the request, based on the allocation score; and transmitting data specifying the content item in response to the request.