Kali Linux (formerly known as BackTrack) is a Debian-based distribution with a collection of security and forensics tools. It features timely security updates, support for the ARM architecture, a choice of four popular desktop environments, and seamless upgrades to newer versions.

Kali Linux has over 600 preinstalled penetration-testing programs, including Armitage (a graphical cyber attack management tool), Nmap (a port scanner), Wireshark (a packet analyzer), John the Ripper (a password cracker), Aircrack-ng (a software suite for penetration-testing wireless LANs), Burp suite and OWASP ZAP web application security scanners.

It was developed by Mati Aharoni and Devon Kearns of Offensive Security through the rewrite of BackTrack, their previous information security testing Linux distribution based on Knoppix. Originally, it was designed with a focus on kernel auditing, from which it got its name Kernel Auditing LInux. The name is sometimes incorrectly assumed to come from Kali the Hindu goddess. The third core developer, Raphaël Hertzog, joined them as a Debian expert. Kali Linux is based on the Debian Testing branch. Most packages Kali uses are imported from the Debian repositories.

Kali Linux popularity grew when it was featured in multiple episodes of the TV series Mr. Robot. Tools highlighted in the show and provided by Kali Linux include Bluesniff, Bluetooth Scanner (btscanner), John the Ripper, Metasploit Framework, nmap, Shellshock, and Wget. With version 2019.4 in November 2019, the default user interface was switched from GNOME to Xfce, with a GNOME version still available.