This event is organized by the GDR XFEL and aims to present the new possibilities offered by free electron lasers emitting in the X-ray regime (XFEL) for the study of condensed matter, matter in extreme conditions and dense plasmas. The courses and presentations are aimed at an audience with little knowledge of FEL sources (PhD students, post-docs, researchers) and interested in the potential of these large instruments. The school will devote a large place to the operation of the source, the techniques and the methods used. The scientific questions addressed will concern condensed matter (atomic, electronic and magnetic structure), the physics of dense plasmas and matter in extreme conditions. Each thematic session will be preceded by a presentation of the highlights obtained on the XFEL, always taking care to place them in a historical context in relation to results obtained on synchrotrons and on laser installations.