  The Cosmic Lens All-Sky Survey (CLASS) is an international collaborative
program which has obtained high-resolution radio images of over 10000
flat-spectrum radio sources in order to create the largest and best studied
statistical sample of radio-loud gravitationally lensed systems. With this
survey, combined with detailed studies of the lenses found therein, constraints
can be placed on the expansion rate, matter density, and dark energy (e.g.
cosmological constant, quintessence) content of the Universe that are
complementary to and independent of those obtained through other methods. CLASS
is aimed at identifying lenses where multiple images are formed from compact
flat-spectrum radio sources, which should be easily identifiable in the radio
maps. Because CLASS is radio-based, dust obscuration in lensing galaxies is not
a factor, and the relative insensitivity of the instrument to environmental
conditions leads to nearly uniform sensitivity and resolution over the entire
survey. In four observing seasons from 1994-1999, CLASS has observed 13783
radio sources with the VLA at 8.4 GHz at 0.2 arcsecond resolution. When
combined with the JVAS survey, the CLASS sample contains over 16,000 images. A
complete sample of 11685 flat-spectrum sources was observed, selected from GB6
catalogue at 4.85 GHz and the NVSS at 1.4 GHz. So far, CLASS has found 16 new
gravitational lens systems, and the JVAS/CLASS survey contains a total of 22
lenses. (Abridged)
