  Coded mask instruments have been used in high-energy astronomy for the last
forty years now and designs for future hard X-ray/low gamma-ray telescopes are
still based on this technique when they need to reach moderate angular
resolutions over large field of views, particularly for observations dedicated
to the, now flourishing, field of time domain astrophysics. However these
systems are somehow unfamiliar to the general astronomers as they actually are
two-step imaging devices where the recorded picture is very different from the
imaged object and the data processing takes a crucial part in the
reconstruction of the sky image. Here we present the concepts of these optical
systems applied to high-energy astronomy, the basic reconstruction methods
including some useful formulae and the trend of the expected and observed
performances as function of the system designs. We review the historical
developments and recall the flown space-borne coded mask instruments along with
the description of a few relevant examples of major successful implementations
and future projects in space astronomy.
