  Technology has advanced to the point that it is possible to image the entire
sky every night and process the data in real time. The sky is hardly static:
many interesting phenomena occur, including variable stationary objects such as
stars or QSOs, transient stationary objects such as supernovae or M dwarf
flares, and moving objects such as asteroids and the stars themselves. Funded
by NASA, we have designed and built a sky survey system for the purpose of
finding dangerous near-Earth asteroids (NEAs). This system, the "Asteroid
Terrestrial-impact Last Alert System" (ATLAS), has been optimized to produce
the best survey capability per unit cost, and therefore is an efficient and
competitive system for finding potentially hazardous asteroids (PHAs) but also
for tracking variables and finding transients. While carrying out its NASA
mission, ATLAS now discovers more bright ($m < 19$) supernovae candidates than
any ground based survey, frequently detecting very young explosions due to its
2 day cadence. ATLAS discovered the afterglow of a gamma-ray burst independent
of the high energy trigger and has released a variable star catalogue of
5$\times10^{6}$ sources. This, the first of a series of articles describing
ATLAS, is devoted to the design and performance of the ATLAS system. Subsequent
articles will describe in more detail the software, the survey strategy,
ATLAS-derived NEA population statistics, transient detections, and the first
data release of variable stars and transient lightcurves.
