  This is a short and biased review of gravitational lensing with emphasis on
the radio and especially VLBI aspects. We briefly explain the basic idea and
give a short sketch of the discovery of the first lens, before we more
systematically discuss the general fields that can be studied with lensing. We
intentionally omit the details the average lensing expert would like to see and
instead try to give a very general overview addressed to the radio astronomer
working in some other field. The lens B0218+357 is presented as an example to
show many aspects of lensing in a case that already has led to interesting
results but still has additional potential for the future.
