eROSITA is the core instrument on the Russian Spectrum-Roentgen-Gamma mission. From 2013 on eROSITA is expected to deeply survey the entire X-ray sky. The driving science is the detection of 100.000 distant galaxy clusters in order to study the large scale structure in the Universe and test cosmological models including Dark Energy. In addition, eROSITA will detect about 3 million Active Galaxies, widening our view on the evolution of supermassive black holes. eROSITA will also provide new clues on other astrophysical topics, like X-ray binaries and the diffuse galactic emission.