The main strengths of ROSAT  are its high quality imaging of extended sources and its large sensitivity for the detection of point sources. In the all-sky survey the diffuse X-ray emission was mapped with a resolution of 12 arcmin . The number of point sources detected in the survey is ≈ 80 000 . In addition more than 80 000 sources have been found in the ≈ 6 500 fields observed with the PSPC and HRI until now in the course of the pointed programme. A wealth of information about source variabilities has been extracted both from the all-sky survey and from the pointed observations, as well as from a combination of both.