Multics (Multiplexed Information and Computing Service) was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts. The last known running Multics installation was shut down on October 30, 2000, at the Canadian Department of National Defence in Halifax, Nova Scotia, Canada.

Initial planning and development for Multics started in 1964. Originally it was a cooperative project led by MIT (with Fernando Corbató) along with General Electric and Bell Labs. Bell Labs pulled out in 1969, and in 1970 GE's computer business including Multics was taken over by Honeywell.

Multics was conceived as a commercial product for GE, and became one for Honeywell, but not a very successful one. Due to its many novel and valuable ideas, Multics had a significant impact in the computer field even though it was derided by some critics at the time.

Multics had numerous features intended to result in high availability so that it would support a computing utility similar to the telephone and electricity services. Modular hardware structure and software architecture were used to achieve this. The system could grow in size by simply adding more of the appropriate resource — computing power, main memory, disk storage, etc. Separate access control lists on every file provided flexible information sharing, but complete privacy when needed. Multics had a number of standard mechanisms to allow engineers to analyze the performance of the system, as well as a number of adaptive performance optimization mechanisms.