RFID technology is gaining adoption on an increasing scale for tracking and monitoring purposes. Wide deployments of RFID devices will soon generate an unprecedented volume of data. Emerging applications require the RFID data to be filtered and correlated for complex pattern detection and transformed to events that provide meaningful, actionable information to end applications. In this work, we design and develop SASE, a complex event processing system that performs such datainformation transformations over real-time streams. We design a complex event language for specifying application logic for such transformation, devise new query processing techniques to efficiently implement the language, and develop a comprehensive system that collects, cleans, and processes RFID data for delivery of relevant, timely information as well as storing necessary data for future querying. We demonstrate an initial prototype of SASE through a real-world retail management scenario.