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 2013. eROSITA will perform an all-sky survey lasting four years, followed by a phase of three years for pointed observations. eROSITA consists of seven identical Mirror Modules, each equipped with 54 Wolter-I shells with an outer diameter of 360 mm. This would provide an effective area of similar to 1500 cm(2) at 1.5 keV and an on axis PSF HEW of 15 arcsec resulting in an effective angular resolution of 28 arcsec averaged over the field of view. In the focus of each mirror module, a fast frame-store pn-CCD provides a field of view of 1 in diameter. In this paper we report on the instrument development and its status only.