Infomaster is an information integration system. It provides integrated access to distributed, heterogeneous information sources, thus giving its users the illusion of a centralized, homogeneous information system. Infomaster is the first such system that is able to handle arbitrary positive relational algebra user queries and database descriptions. It is able efficiently to use integrity constraints and local completeness information for optimization. The system has been deployed in a wide variety of application areas, including engineering, logistics, and electronic commerce. This article provides a much requested overview of the query processing method used by Infomaster.