Cosmological Microlensing
  Variability in gravitationally lensed quasars can be due to intrinsic fluctuations of the quasar or due to ``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. In particular, multi-waveband observations are useful for this goal.   In this review recent theoretical progress as well as observational evidence for quasar microlensing over the last few years will be summarized. Comparison with numerical simulations will show where we stand. Particular emphasis will be given to the questions 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 will be given as a conclusion/outlook. 
Joachim Wambsganss (Universitaet Potsdam and MPI fuer   Gravitationsphysik, Golm)