  This review forms the Weak Lensing part of the Saas-Fee Advanced Course on
Gravitational Lensing. It describes the basicsm applications and results of
weak lensing. Contents: (1) Introduction (2) The principles of weak
gravitational lensing (3) Observational issues and challenges (4) Clusters of
galaxies: Introduction, and strong lensing (5) Mass reconstructions from weak
lensing (6) Cosmic shear -- lensing by the LSS (7) Large-scale structure
lensing: results (8) The mass of, and associated with galaxies (9) Additional
issues in cosmic shear (10) Concluding remarks.
