AT may mean:

• Aarne-Thompson classification system
• AT Field, a force field in the anime Neon Genesis Evangelion
• Acceptance test
• Air Trecks, fictional inline skates in the manga and anime Air Gear
• Alimentary tract
• Atmel Corporation
• Anaerobic threshold
• Análisis Transaccional (Spanish "Transactional analysis"), a psychological method
• Anguilla, a World Meteorological Organization country code
• Anti-tamper, resistance to tamper of a product, package, or system
• Anti-tank weapons
• Counter-terrorism, also known as "anti-terrorism" (AT)
• Appalachian Trail
• Aphex Twin
• Appropriate Technology
• Ashmore and Cartier Islands (FIPS 10-4 territory code and obsolete NATO country code
• Assistive technology
• IBM Personal Computer/AT
• AT form factor for motherboards and computer cases
• The common name for the 5-pin DIN keyboard connector
• Ataxia telangiectasia, an immunodeficiency disorder
• Atchison County, Kansas (county code)
• Attack Team (game), a 2000 computer game
• Audit trail
• Austria's ISO, 2-letter country code
• .at is the Internet country code top-level domain for Austria
• Automatic transmission
• Hayes command set for computer modems (all commands begin with the characters "AT")
• Royal Air Maroc (IATA airline designator)
• Abel Tasman, a seventeenth century Dutch explorer and merchant

At may mean:

• Ampere-turn (symbol), an International System of Units (MKS) unit of magneto-motive force
• Astatine, the chemical symbol for the element
• a subdivision equal to 1/100 of a Kip, the unit of currency in Laos

aT may mean:

• attotesla, an SI unit of magnetic flux density

at may mean:

• "@", the punctuation symbol now typically used in e-mail addresses
• at (Unix), to schedule tasks to be performed at a later date
• technical atmosphere (symbol), physical unit of pressure