  The Atacama Cosmology Telescope was designed to measure small-scale
anisotropies in the Cosmic Microwave Background and detect galaxy clusters
through the Sunyaev-Zel'dovich effect. The instrument is located on Cerro Toco
in the Atacama Desert, at an altitude of 5190 meters. A six-meter off-axis
Gregorian telescope feeds a new type of cryogenic receiver, the Millimeter
Bolometer Array Camera. The receiver features three 1000-element arrays of
transition-edge sensor bolometers for observations at 148 GHz, 218 GHz, and 277
GHz. Each detector array is fed by free space mm-wave optics. Each frequency
band has a field of view of approximately 22' x 26'. The telescope was
commissioned in 2007 and has completed its third year of operations. We discuss
the major components of the telescope, camera, and related systems, and
summarize the instrument performance.
