TOS (The Operating System, sometimes called Tramiel Operating System) is the operating system that powers every Atari ST, STE, TT and Falcon computer.
TOS combines a compact kernel (GEMDOS, providing file and process handling similar to CP/M and MS-DOS) with GEM (Graphical Environment Manager), the graphical desktop and windowing system that gives the Atari ST its familiar look with icons, windows and drop-down menus.
TOS 1.00 shipped with the earliest STs, while later revisions like TOS 1.04 and TOS 1.62 added bug fixes and Blitter support. TOS 2.06, used on the Mega STE and commonly flashed onto expanded Mega STs today, added support for higher resolutions and better compatibility overall.
Modern replacements and alternatives like EmuTOS (a free, open-source reimplementation) and Kaos TOS are frequently used on modified machines to fix compatibility issues, add features, or simply avoid depending on scarce original ROM chips. PP's improved iTOS 1.04 is another popular option, adding real FAT16 filesystem support for much better hard disk handling. These are exactly the kind of setups used across the machines documented on this site.