Requirement Engineering plays a key role in developing complex space systems successfully. How to achieve efficient and effective information exchange during the whole life cycle of a space system is a challenge, which can be tackled by realizing
between involved partners (as presented in ECSS-E-TM-10-23A ). Using academic works related to fact based modelling (FBM), the European Space Agency (ESA) has initiated the development of a new knowledge management system (called Fact-based Modeling Unifying System or FAMOUS) to provide means to tackle semantic interoperability. The Agency has also organized an international working group of FBM experts to support this objective. In this paper, two Requirement Engineering use cases currently developed at ESA will be presented: one related to requirement analysis for spacecraft on-board software and the other one to requirement management for human spaceflight missions.