Corporations employ data mining to analyze operations, find trends in recorded information, and look for new opportunities. Libraries are no different. Librarians manage large stores of data--about collections and usage, for example--and they also want to analyze this data to serve their users better. Analysts use data mining to query a data warehouse for patterns that a human couldn't manually spot. For example, an online vendor or credit card company can map product types against zip codes, shipping preferences, time of day, and card expiration dates to flag potentially fraudulent purchases. Data mining is performed in a data warehouse. While an operational database system like an integrated library system (ILS) is optimized for processing transactions (circulation, purchases, cataloging, etc), a data warehouse is optimized for analysis. This makes it easier to find patterns and avoid bogging down the transactional system. This article discusses data mining and how libraries are now getting into the act.