  The determination of the Hubble parameter H_0 is probably one of the most
important applications of quasar lensing. The method, based on the measurement
of the so-called ``time-delay'' between the lensed images of distant sources,
e.g., quasars, and on detailed mass modeling of the potential well responsible
for the multiple images, yields an accuracy at least comparable with other
techniques and that can be improved further with high precision observations,
as can be obtained with intrumentation of constantly increasing quality. The
basics of the ``time-delay'' method are described, and the emphasis is put on
the observational constraints available to the astrophysicist in order to
implement the method and to derive an accurate value for H_0, independent of
any standard candle or any strong prior on the other cosmological parameters.
