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 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 identified domains using the first query to identify resources associated with the identified domains; determining one or more queries based on data reflecting past search queries, where each of the one or more determined queries resulted in one or more of the identified resources being returned as part of a search result; determining keywords based on the one or more determined queries; and transmitting or storing the keywords for use in impression allocation decisions.