The Network Livermore Timesharing System (NLTSS, also sometimes the New Livermore Time Sharing System and internally as LINOS, the LINCS Interactive Network Operating System) is an operating system that was actively developed at Lawrence Livermore Laboratory (LLL) (now Lawrence Livermore National Laboratory, LLNL) from 1979 until about 1988, though it continued to run production applications and be supported and even, in some cases, extended until 1995. A previous operating system, the Livermore Time Sharing System had been developed over a decade earlier at LLL.

NLTSS ran initially on a CDC 7600 computer, but only ran production from about 1984 until 1995 on Cray computers including the Cray-1, Cray X-MP, and Cray Y-MP models.

The NLTSS operating system was unusual in many respects and unique in some.

NLTSS was a microkernel message passing system. It was unique in that only one system call was supported by the kernel of the system. That system call, which might be called "communicate" (it didn't have a name because it didn't need to be distinguished from other system calls) accepted a list of "buffer tables" (e.g., see The NLTSS Message System Interface) that contained control information for message communication – either sends or receives. Such communication, both locally within the system and across a network was all the kernel of the system supported directly for user processes. The "message system" (supporting the one call and the network protocols) and drivers for the disks and processor composed the entire kernel of the system.

NLTSS is a capability-based security client–server system. The two primary servers are the file server and the process server. The file server was a process privileged to be trusted by the drivers for local storage (disk storage,) and the process server was a process privileged to be trusted by the processor driver (software that switched time sharing control between processes in the "alternator", handled interrupts for processes besides the "communicate" call, provided access to memory and process state for the process server, etc.).

NLTSS was a true network operating system in that its resource requests could come from local processes or remote processes anywhere on the network and the servers didn't distinguish them. A server's only means to make such distinctions would be by network address and they had no reason to make such distinctions. All requests to the servers appeared as network requests.

Communication between processes in NLTSS by convention used the Livermore Interactive Network Communication System (LINCS) protocol suite, which defined a protocol stack along the lines of that defined by the OSI reference model. The transport level protocol for NLTSS and LINCS was named Delta-T. At the presentation level, LINCS defined standards for communicating numbered parameters as tokens (e.g., integers, capabilities, etc.) that were stored in a session level record for processing in a remote procedure call sort of mechanism.

The notion of a "user" was only rather peripherally defined in NLTSS. There was an "account server" that kept track of which users were using which resources (e.g., requests to create objects such as file or processes required such an account capability). Access control was entirely managed with capabilities (communicable authority tokens).