  Imaging Atmospheric Cherenkov Telescopes (IACTs) have resulted in a
breakthrough in very-high energy (VHE) gamma-ray astrophysics. While early IACT
installations faced the problem of detecting any sources at all, current
instruments are able to see many sources, often over more than two orders of
magnitude in energy. As instruments and analysis methods have matured, the
requirements for calibration and modelling of physical and instrumental effects
have increased. In this article, a set of Monte Carlo simulation tools is
described that attempts to include all relevant effects for IACTs in great
detail but aims to achieve this in an efficient and flexible way. These tools
were originally developed for the HEGRA IACT system and later adapted for the
H.E.S.S. experiment. Their inherent flexibility to describe quite arbitrary
IACT systems makes them also an ideal tool for evaluating the potential of
future installations. It is in use for design studies of CTA and other
projects.
