Methods, systems, and apparatus, including medium-encoded computer program products, for searching a data set and returning search results organized in a hierarchy of categories are disclosed. A set of categories is provided for organizing a set of search results, wherein each category is associated with one or more search results. The set of search results is organized into a hierarchy of categories, the hierarchy including at least one category from the set of categories. At least a portion of the hierarchy of categories is displayed and a user request to modify the hierarchy of categories is received. The hierarchy of categories is modified in accordance with the user request.