Systems and methods for query expansion are described. In one aspect, new terms are extracted from a newly submitted query. Terms to expand the new terms are identified to a relevant document list. The expansion term are identified at least in part on the new terms and probabilistic correlations from information in a query log. The query log information includes one or more query terms and a corresponding set of document identifiers (IDs). The query terms were previously submitted to a search engine. The document IDs represent each document selected from a list generated by the search engine in response to searching for information relevant to corresponding ones of the query terms.