  I shall outline the basic principles and some observational aspects of weak
gravitational lensing, and discuss several applications of this powerful tool
in observational cosmology. It will be explained why the applications have been
restricted to optical observations up to now, and why SKA is going to change
this. I conclude with a few general remarks on a comparison between SKA and the
NGST, both being facilities which will provide a tremendous step forward in
radio and near-IR astronomy, respectively, into completely unknown territory.
