Avast! antivirus is an antivirus program developed by ALWIL Software a.s. based in Prague, Czech Republic and was first released in 1988. It is based on a central scanning engine which is certified by ICSA Labs and West Coast Lab's Checkmark process and incorporates anti-spyware technology, also certified by West Coast Lab's Checkmark process, as well as anti-rootkit and self-protection capabilities. It is a multiple recipient of the Virus Bulletin VB100 Award, for detection of 100% of "in-the-wild" viruses and is a past winner of the Secure Computing Readers' Trust Award.

Avast! Home Edition is the freeware version of avast! antivirus available to Microsoft Windows and Linux users, while avast! Professional Edition is offered to businesses and users that want additional features. avast! Professional Edition contains an enhanced user interface, through which scanning tasks can be scheduled to run automatically, while priority updates are delivered automatically using PUSH update technology. The Professional Edition also has a command line scanner and a script blocker.

Avast! antivirus is one of the most widely used antivirus programs in the world, with more than 80 million users worldwide as of April 2009.