  The purpose of these lecture notes is to describe the gravitational lens
effects in different astrophysical contexts. These notes are voluntarily
focused on the fundamental mechanisms and the basic concepts that are useful to
describe these effects. The observational consequences are presented in more
details in accompanying notes by Y. Mellier.
  The content of these notes is the following. In the first section describe of
the basic mechanisms of gravitational lenses, techniques and approximations
that are usually employed. The second section is devoted to the case of a very
simple deflector, a point-like mass distribution. This corresponds to
microlensing events in which the deflectors are compact objects of a fraction
of a solar mass that may populate the halo of our Galaxy. The last two sections
are devoted to cosmological applications. After a presentation of the
geometrical quantities that are specific to cosmology, I will present the
various phenomena that can be observed in this context. Finally I describe the
weak lensing regime. This is a rapidly developing area that should eventually
allow us to map the mass distribution in the Universe.
