8. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: receiving a first query; receiving a second, follow-up query; determining that a pronoun in the second, follow-up query is an anaphor by determining that the pronoun refers to an entity that is not present in the second, follow-up query; in response to determining that the pronoun in the second, follow-up query is an anaphor, determining that the first query is associated with a plurality of possible entities; generating a plurality of candidate queries, wherein each candidate query of the plurality of candidate queries is generated by replacing the pronoun in the second, follow-up query with a corresponding possible entity; ranking, using at least past query logs stored in a data repository, the plurality of candidate queries; and providing a highest-ranked candidate query of the plurality of candidate queries to a search engine and obtaining search results for the highest-ranked candidate query from the search engine.
8. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: receiving a first query; receiving a second, follow-up query; determining that a pronoun in the second, follow-up query is an anaphor by determining that the pronoun refers to an entity that is not present in the second, follow-up query; in response to determining that the pronoun in the second, follow-up query is an anaphor, determining that the first query is associated with a plurality of possible entities; generating a plurality of candidate queries, wherein each candidate query of the plurality of candidate queries is generated by replacing the pronoun in the second, follow-up query with a corresponding possible entity; ranking, using at least past query logs stored in a data repository, the plurality of candidate queries; and providing a highest-ranked candidate query of the plurality of candidate queries to a search engine and obtaining search results for the highest-ranked candidate query from the search engine. 12. The system of claim 8 , wherein determining that the first query is associated with a plurality of possible entities comprises: analyzing results of the first query for a presence of an entity associated with the anaphor.