ATLAS (A Toroidal LHC ApparatuS) is one of the six particle detector experiments (ALICE, ATLAS, CMS, TOTEM, LHCb, and LHCf) currently being constructed at the Large Hadron Collider (LHC), a new particle accelerator at the European Organization for Nuclear Research (CERN) in Switzerland. When completed, ATLAS will be 46 metres long and 25 metres in diameter, and will weigh about 7,000 tonnes. The project involves roughly 2,000 scientists and engineers at 165 institutions in 35 countries. The construction was scheduled to be completed in June 2007, however is now stated to be mid-2008. The experiment is designed to observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators and might shed light on new theories of particle physics beyond the Standard Model.

The ATLAS collaboration, the group of physicists building the detector, was formed in 1992 when the proposed EAGLE (Experiment for Accurate Gamma, Lepton and Energy Measurements) and ASCOT (Apparatus with Super COnducting Toroids) collaborations merged their efforts into building a single, general-purpose particle detector for the Large Hadron Collider. The design was a combination of those two previous designs, as well as the detector research and development that had been done for the Superconducting Supercollider. The ATLAS experiment was proposed in its current form in 1994, and officially funded by the CERN member countries beginning in 1995. Additional countries, universities, and laboratories joined in subsequent years, and further institutions and physicists continue to join the collaboration even today. The work of construction began at individual institutions, with detector components shipped to CERN and assembled in the ATLAS experimental pit beginning in 2003.

ATLAS is designed as a general-purpose detector. When the proton beams produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies may be produced. Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of signals. This is intended to ensure that, whatever form any new physical processes or particles might take, ATLAS will be able to detect them and measure their properties. Experiments at earlier colliders, such as the Tevatron and Large Electron-Positron Collider, were designed based on a similar philosophy. However, the unique challenges of the Large Hadron Colliderits unprecedented energy and extremely high rate of collisions requir e ATLAS to be larger and more complex than any detector ever built.