  Gravitational lensing is now widely and successfully used to study a range of
astronomical phenomena, from individual objects, like galaxies and clusters, to
the mass distribution on various scales, to the overall geometry of the
Universe. Here we describe and assess the use of gravitational lensing as
``gold standards'' in addressing one of the fundamental problems in astronomy,
the determination of the absolute distance scale to extragalactic objects. This
is commonly parameterized by the Hubble constant, $H_0$, the current expansion
rate of the Universe. The elegance of the underlying geometrical principle of
the gravitational lensing method combined with the recent advances in
observations and modelling makes it a very promising technique for measuring
$H_0$.
