Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for extracting semantic classes and corresponding instances from a collection of text. One aspect of the subject matter described in this specification can be embodied in methods that include the actions of receiving a collection of text; identifying an initial collection of instance-class pairs for the collection of text; clustering a collection of semantically similar phrases using the collection of text; generating, using one or more processors, an extracted collection of instance-class pairs using the initial collection of instance-class pairs and the semantically similar phrase clusters; and storing the extracted collection of instance-class pairs for use in information retrieval.