  We describe a series of new applications of gravitational lenses as
astrophysical and cosmological tools. Such applications are becoming possible
thanks to advances in the quality and quantity of observations. CASTLES
(CfA-Arizona-Space-Telescope-LEns-Survey) {cfa-www.harvard.edu/castles} is an
ongoing project that exploits the sensitivity and resolution of the Hubble
Space Telescope (HST) at optical and infrared wavelengths to study the sample
of over 50 known gravitational lenses. The observational goal of CASTLES is a
uniform sample of multi-band images of all known galaxy-mass lens systems, to
derive precise photometry and astrometry for the lens galaxies, all the known
images, and any source or lens components that might have escaped detection.
With these measurements we are investigating: (1) the properties of dust and of
dark matter in lens galaxies out to z\sim1; (2) the dark matter in lens
galaxies and in their environments; (3) the evolution of lens galaxies; and (4)
the cosmological model, for instance by refining constraints on the Hubble
constant H_0.

Astrophysics