This paper describes an intelligent maintenance system for Supporting Crew OPErations (SCOPE). SCOPE supports the astronauts onboard the ISS and helps them to maximize the availability of complex payload systems. SCOPE detects system failures, guides the isolation of the root causes of failure, and presents the relevant repair procedures in textual and graphical formats. The diagnosis process is a joint astronaut-SCOPE activity: when needed, the system asks the astronaut to perform additional measurements in order to help resolve uncertainties, ambiguities or conflicts in the current payload status model. Usability tests show good user performance and satisfaction. The current SCOPE prototype has been applied to a portable payload for medical experimentation. © 2004 IEEE.