  The physical state of the intergalactic medium can be probed in great detail
with the intervening absorption systems seen in quasar spectra. The properties
of the Hydrogen absorbers depend on many cosmological parameters, such as the
matter-power spectrum, reionisation history, ionising background and the nature
of the dark matter. The spectra also contain metal lines, which can be used to
constrain the star formation history and the feedback processes acting in large
and small galaxies. Simulations have been instrumental in investigating to what
extent these parameters can be unambiguously constrained with current and
future data. This paper is meant as an introduction to this subject, and
reviews techniques and methods for simulating the intergalactic medium.
