Methods, systems, and apparatus, including computer program products, to identify topic words in a collection of documents that includes topic documents related to a topic are disclosed. A reference topic word divergence value based on a document collection and the topic document collection is determined. A candidate topic word divergence value for a candidate topic word is determined based on the document collection and the topic document collection. The candidate topic word is determined to be a topic word if the candidate topic word divergence value is greater than the reference topic word divergence value.