Gravitational Lensing and Cluster of Galaxies
  Multiple images, giant arcs, Einstein cross, fold, cusp, lip, caustics, critical lines, sources, mapping, time delay, arclets, weak shear, magnification bias, depletion, ellipticities, polarization, smearing, convergence, kernel, mass reconstruction, sub-structure, dark halos, natural telescope, amplification, distant galaxies, faintest sources ... This is a non exhaustive list of favorite terms used when using gravitational lensing in clusters of galaxies. I will introduce here lensing in clusters as a useful tool in modern observational cosmology. I will give a summary of what we are learning in terms of cluster mass distribution in the strong and weak regime, and what information we will gain in terms of the cluster physics. I will underline the benefit of using cluster-lenses as natural telescopes to probe the distant Universe. 
Jean-Paul Kneib (OMP, Toulouse)