We report on the development of a generic X‐ray instrument simulator to be used in simulations of future X‐ray missions. Based on a Monte Carlo approach the code generates photon events for sources in an X‐ray source catalogue such as the ROSAT all sky survey or the XMM‐Newton slew surveys and then models the imaging and detection process based on the available calibration files (e.g., point spread functions for the imaging). The output of the program are event lists, which can be analysed using standard software such as xselect. Due to its modular concept the simulation software can be easily adapted to different concepts of imaging detectors.As examples for the potential use of the simulation we present our studies for eROSITA and show results of simulations of the detector performance for the High Time Resolution Spectrometer and the Wide Field Imager on the International X‐ray Observatory.