Abstract: User behavior data can be used with language-specific rule sets to generate stemming databases useful for such tasks as indexing and search query processing. The terms contained in user queries, as well as user behavior with respect to those queries or results returned for those queries, can be analyzed to determine a relative measure (e.g., relative frequency) of various forms of those terms. When generating a stemming database, language-specific rule sets can be used to determine appropriate stemming rules, and where more than one potential rule is identified the user behavior data can be used to select what is likely the appropriate rule, at least for the respective environment. Whitelists or other such components can be used to handle specific or irregular forms that do not follow the general rules or otherwise are exceptions that might not otherwise be processed correctly.