Avast! is an antivirus computer program developed by AVAST Software a.s. (formerly known as ALWIL Software a.s.), a company based in Prague, Czech Republic. It was first released in 1988 although back then it was just a tool to remove the Vienna malware. Avast! is based on a central scanning engine that 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 its detection of 100% of "in-the-wild" viruses, and is a past winner of the Secure Computing Readers' Trust Award. The name Avast is a acronym of "Anti-Virus – Advanced Set". Its trademarked name is styled in all lower-case and includes an exclamation mark, and a slogan 'be free'. Avast! Free Antivirus is the freeware version of the Avast! antivirus software available to Microsoft Windows and Linux users, while Avast! Pro Antivirus is offered to businesses and users who want additional features. Priority updates are delivered automatically using PUSH update technology in Avast! Pro Antivirus. Avast! Pro Antivirus also has a command line scanner and a script blocker; however, in version 6, the script blocker is available to users of the free version. Avast! Antivirus is a widely used antivirus program, with over 165 million users worldwide as of October 2011.

Currently, over 196 million users trust Avast!