This specification describes technologies relating to generation of keywords. In general, one aspect of the subject matter described in this specification can be embodied in methods that include receiving structured data describing a content item, the structured data indicating a category for the content item. The methods may further include searching domains associated with the category using a first query, formed based on data elements in the structured data, to identify resources associated with the identified domains. The methods may further include 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, and determining keywords based on the one or more determined queries. The methods may further include transmitting or storing the keywords for use in impression allocation decisions.