eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is scheduled for launch in late 2012. eROSITA is fully approved and funded by the German Space Agency DLR and the Max-Planck-Society. The design driving science is the detection of 50-100 thousands Clusters of Galaxies up to redshift z∼1.3 in order to study the large scale structure in the Universe and test cosmological models, especially Dark Energy. This will be accomplished by an all-sky survey lasting for four years plus a phase of pointed observations. eROSITA consists of seven Wolter-I telescope modules, each equipped with 54 Wolter-I shells having an outer diameter of 360 mm. This would provide and effective area at 1.5 keV of ∼1500cm2 and an on axis PSF HEW of 15″ which would provide an effective angular resolution of 25″-30″. In the focus of each mirror module, a fast frame-store pn-CCD will provide a field of view of 1° in diameter for an active FOV of ∼0.83deg2. At the time of writing the instrument development is currently in phase C/D.