  We give an overview of the SImulation of X-ray TElescopes (SIXTE) software
package, a generic, mission-independent Monte Carlo simulation toolkit for
X-ray astronomical instrumentation. The package is based on a modular approach
for the source definition, the description of the optics, and the detector type
such that new missions can be easily implemented. The targets to be simulated
are stored in a flexible input format called SIMPUT. Based on this source
definition, a sample of photons is produced and then propagated through the
optics. In order to model the detection process, the software toolkit contains
modules for various detector types, ranging from proportional counter and
Si-based detectors, to more complex descriptions like transition edge sensor
(TES) devices. The implementation of characteristic detector effects and a
detailed modeling of the read-out process allow for representative simulations
and therefore enable the analysis of characteristic features, such as for
example pile-up, and their impact on observations. We present an overview of
the implementation of SIXTE from the input source, the imaging, and the
detection process, highlighting the modular approach taken by the SIXTE
software package. In order to demonstrate the capabilities of the simulation
software, we present a selection of representative applications, including the
all-sky survey of eROSITA and a study of pile-up effects comparing the
currently operating XMM-Newton with the planned Athena-WFI instrument. A
simulation of a galaxy cluster with the Athena- X-IFU shows the capability of
SIXTE to predict the expected performance of an observation for a complex
source with a spatially varying spectrum and our current knowledge of the
future instrument.
