The primary science driver for eROSITA is to study dark energy with about 100,000 galaxy clusters. eROSITA will survey the full sky eight times at X-ray wavelengths. It will be launched as the main instrument onboard the Spektr-RG satellite mission end of 2017. Technical and scientific descriptions as well as forecasts on eROSITA's cosmological constraining power can be found, e.g., in Predehl et al. (2010), Merloni et al. (2012), and Pillepich et al. (2012), respectively.