Quasar Lensing
  Massive structures, such as galaxies, act as strong gravitational lenses on background sources. When the background source is a quasar, several lensed images are seen, as magnified or de-magnified versions of the same object. The detailed study of the image configuration and the measurement of ``time-delays'' between the images yield estimates of the Hubble parameter H_0. We describe in a simple way the phenomenon of strong lensing and review recent progress made in the field, including microlensing by stars in the main lensing galaxy. 
F. Courbin (Liege, Belgium), P. Saha (Queen Mary and Westfield   College, UK), P.L. Schechter (MIT, USA)