  In these lectures I give an overview of gravitational lensing, concentrating
on theoretical aspects, including derivations of some of the important results.
Topics covered include the determination of surface mass densities of
intervening lenses, as well as the statistical analysis of distortions of
galaxy images by general inhomogeneities (cosmic shear), both in 2D projection
on the sky, and in 3D where source distance information is available. 3D mass
reconstruction and the shear ratio test are also considered, and the
sensitivity of observables to Dark Energy is used to show how its equation of
state may be determined using weak lensing. Finally, the article considers the
prospect of testing Einstein's General Relativity with weak lensing, exploiting
the differences in growth rates of perturbations in different models.}
  \abstract{In these lectures I give an overview of gravitational lensing,
concentrating on theoretical aspects, including derivations of some of the
important results. Topics covered include the determination of surface mass
densities of intervening lenses, as well as the statistical analysis of
distortions of galaxy images by general inhomogeneities (cosmic shear), both in
2D projection on the sky, and in 3D where source distance information is
available. 3D mass reconstruction and the shear ratio test are also considered,
and the sensitivity of observables to Dark Energy is used to show how its
equation of state may be determined using weak lensing. Finally, the article
considers the prospect of testing Einstein's General Relativity with weak
lensing, exploiting the differences in growth rates of perturbations in
different models.
