X-rays are generated by matter under extreme physical conditions. They occur in the vicinity of compact objects, in the coronae of active stars, in the centers of active galaxies or in the hot intergalactic matter in clusters of galaxies. Since X-rays from space are absorbed in the Earth's atmosphere, space-borne observatories like XMM-Newton or eROSITA are needed for their detection. The physical interpretation however requires observations over the whole electromagnetic spectral range and we are making use of the AIP resources STELLA and LBT (among many others). Current science topics are compact stellar sources and AGN, in particular those found in the ongoing all-sky surveys eROSITA.