  There are two possible causes of variability in gravitationally lensed
quasars: intrinsic fluctuations of the quasar and ``microlensing'' by compact
objects along the line of sight. If disentangled from each other,
microlens-induced variability can be used to study two cosmological issues of
great interest: the size and brightness profile of quasars on one hand, and the
distribution of compact (dark) matter along the line of sight on the other.
Here we present a summary of recent observational evidence for quasar
microlensing as well as of theoretical progress in the field. Particular
emphasis is given to the questions which microlensing can address regarding the
search for dark matter, both in the halos of lensing galaxies and in a
cosmologically distributed form. A discussion of desired observations and
required theoretical studies is presented as a conclusion/outlook.
