RT-11 (for Run Time or Real Time) was a real-time operating system for the DEC PDP-11. It was a single-user operating system which did not support multitasking although RT-11 could be configured to support a single "foreground" task running concurrently with a single "background" task. RT-11 was small and could be generated by the user, meaning a sufficiently skilled user with the source code could bootstrap their system from being without any OS into having a functional RT-11 installation. The most common console was a printing terminal, but DEC also supported a VT-11 display device, a vector graphics terminal with a graphic character generator for displaying text, and a light pen. A common program run on the system was "Lunar Lander." You adjusted the thrust and angle of the lander as it landed on the moon. If you landed in the right spot a little man would get out of the lander and plant a flag on the moon.

The RT-11 operating system could be stored on a 2.5MB removable hard disk platter, or quickly booted from an 8 inch floppy disk. The system supported a real time clock, printing terminal, VT-11 vector graphic unit, 16 channel 100KHz A/D converter with 2 channel D/A, 9600 baud serial port, 16 bit bidirectional boards, etc. The file system used an 6.3 file name format (as opposed to MS-DOS 8.3 file name format). An assembly language and a FORTRAN compiler were available from DEC; various compilers were available from other sources. One source of the simplicity (and speed) of RT-11 was the disk architecture; all files were contiguous, meaning a program could initiate file I/O through the operating system and thereafter access the hardware directly. This also meant the disk had to periodically "squeezed" to defragment unused storage.

Later, when DEC introduced the LSI computers, they were able to run RT-11 from an 8 inch floppy disk. Several years later, DEC introduced RSX-11, a multiuser, multitasking operating system, but RT-11 remained the operating system of choice for data acquisition systems where real time response was required.

RT-11 also ran on DEC-10's, DEC 11/34's to DEC 11/60's and the PDT-11 .

S&H Computing developed the TSX-Plus OS, which was essentially a multi-user, multi-processing extension of RT-11.