The Slitless Simulation package aXeSIM Introduction As part of the ST-ECF support for the spectroscopic modes of Wide Field Camera 3 (WFC3), the slitless spectroscopy group of the ST-ECF has developed a dedicated simulation package applicable to WFC3. Whilst the package was initiated for exploitation of WFC3 slitless grism modes, it is equally applicable to other slitless spectroscopy modes of HST, such as ACS and NICMOS.