Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for generating query suggestions based on search data. In one aspect, a method includes receiving, by one or more computers, a first query, determining query refinements based on the first query, generating, from the query refinements, refinement clusters, each refinement cluster corresponding to a particular topic and each refinement cluster including query refinements that are determined to belong to the particular topic to which the refinement cluster corresponds, ranking the refinement clusters, and selecting the refinement cluster that is highest in the ranking relative to other refinement clusters in the ranking as a first search refinement cluster for the first query.