While about 300 SNRs are known in our Galaxy today, the number of SNRs predicted by stellar evolution models is 4–6 times higher, indicating an observational bias due to selection effects. Using eRASS we will search for these missing SNRs in the X-ray band and study the spectrum and morphology of the 70 highly significant SNR candidates of the ROSAT all-sky survey. We will perform a systematic study of the entire population of SNRs in our Milky Way and the Magellanic Clouds and investigate environmental effects on the emission as well as on the evolution of SNRs. We expect to detect previously unknown, highly absorbed extended X-ray SNRs or those with X-ray synchrotron emission. The X-ray emission of the latter will reveal the properties of the shock and the magnetic fields, which are crucial for the understanding of the acceleration of cosmic rays in SNR shocks.