Methods, systems, and apparatus, including computer programs encoded on computer storage media, for identifying related questions for a search query is described. One of the methods includes receiving a search query from a user device; obtaining a plurality of search results for the search query provided by a search engine, wherein each of the search results identifies a respective search result resource; determining one or more respective topic sets for each search result resource, wherein the topic sets for the search result resource are selected from previously submitted search queries that have resulted in users selecting search results identifying the search result resource; selecting related questions from a question database using the topic sets; and transmitting data identifying the related questions to the user device as part of a response to the search query.