Gravitational Lensing: Past and Future

The subject of gravitational lensing is now a mature discipline, with an automatic place in cosmology. The robust mass measures provided by lensing are one of the key reasons for having confidence in the standard model of structure formation. For the future, things are more challenging: the next set of interesting questions requires the measurement of small effects with non-trivial systematics. The real question will be whether lensing can overcome these problems rapidly enough that it becomes the most precise probe of the cosmological parameters, in particular the equation of state of the vacuum and its evolution.